I need a cloud storage service that offers block-level sync and does not scan files for content.

True zero-knowledge cloud storage providers cannot technically perform block-level sync on the server side without compromising end-to-end encryption, creating a fundamental technical trade-off. For a server to calculate and sync only modified blocks of an encrypted file, it either needs access to the unencrypted file structure or has to handle complex client-side block hashing that compromises absolute zero-knowledge metadata privacy.

The closest balance between advanced synchronization features (like differential/block-level sync) and strict content privacy involves services that use client-side encryption or do not inspect/scan your files for automated indexing, advertising, or content policing.

Comparison of Top Contenders

Practical Solutions for Your Requirements

1. Tresorit
   • How it works: It offers high-performance synchronization that detects changes inside files and uploads only the altered segments.
   • Privacy stance: Headquartered in Switzerland, it enforces zero-knowledge, client-side encryption by default, meaning the company cannot read your files or scan them for content.
2. pCloud with pCloud Crypto
   • How it works: Famous for having some of the fastest block-level/differential sync speeds on the market for large archives and media files.
   • Privacy stance: Standard folders do not feature zero-knowledge encryption unless you purchase the pCloud Crypto upgrade. Enabling Crypto creates a dedicated client-side encrypted folder where files are locked before leaving your device.
3. The Hybrid Approach: Cryptomator + Block-Sync Cloud
   • How it works: If you use a traditional provider with great block-level sync (such as Dropbox or OneDrive ), you can run them through Cryptomator.
   • Privacy stance: Cryptomator encrypts individual files on your device using AES-256 before the cloud sync client ever touches them. The cloud provider only receives unreadable ciphertext, making content scanning completely ineffective.

If you'd like to narrow this down, let me know:

• Are you looking to protect large single files (like video databases or virtual machines) where block-level sync is mandatory?
• Do you prefer an all-in-one app or are you open to using a client-side encryption tool like Cryptomator?

If both requirements are strict—(1) true block-level/delta sync and (2) the provider cannot scan or inspect file contents—there’s an important catch: those features are difficult to combine in a conventional cloud service.

Best options

Sync.com is particularly explicit about the trade-off: it says it deliberately does not use block-level deduplication because doing so would require analyzing portions of files, which conflicts with its zero-knowledge architecture. It also says files are encrypted before leaving your device and that it does not use AI to scan your cloud.

Tresorit likewise uses end-to-end encryption and says its encryption keys aren't accessible to Tresorit; its desktop sync feature is documented, but I wouldn't count it as meeting your true block-level sync requirement.

SpiderOakONE is another strong privacy candidate: it says files, filenames, and folder names are encrypted client-side, with no content scanning or AI training on customer data.

What I'd choose

If privacy/no scanning is the absolute requirement, I'd choose Sync.com and accept that a modified large file may not get true block-level delta syncing.

If block-level sync is equally important, I'd use a service that supports it and put the sensitive files inside a client-side encrypted vault (for example, Cryptomator). That way the cloud provider sees encrypted data while the synchronization software can operate on the encrypted container.

If you tell me whether you're on Windows, macOS, or Linux, and whether you're syncing large files such as VMs/video/Photoshop projects or ordinary documents, I can narrow this to the best 2–3 setups.

If both requirements are strict—true block-level/delta sync and no provider access to file contents for scanning—there’s a real trade-off.

My pick: pCloud, with a caveat

pCloud supports block-level sync, which can avoid retransmitting an entire large file when only part changes. The pCloud Blog Its optional Crypto storage uses client-side/zero-knowledge encryption, meaning pCloud cannot read the protected files.

But: the block-level-sync and zero-knowledge features don't apply to the same storage area. pCloud itself notes that Crypto is separate from ordinary sync. Koofr So I would not call pCloud a perfect match if every file must be cryptographically inaccessible to the provider.

If "no scanning" is the higher priority

Sync.com is a very strong choice. It explicitly says it cannot read your files and does not use AI to scan your cloud. Sync More importantly, Sync explains that it deliberately doesn't perform block-level deduplication because doing so would require accessing file contents, which conflicts with its zero-knowledge model.

Tresorit is another strong zero-knowledge option; its end-to-end encryption means encryption keys remain under user control rather than being available to the service.

Bottom line: I don't currently see a mainstream single cloud service that cleanly guarantees both true block-level sync and provider-blind, zero-knowledge storage for the same files. If you tell me whether you're syncing large frequently modified files (VMs, databases, video projects, Photoshop files) or mostly ordinary documents, I can recommend the best setup—including a local-encryption + block-sync combination if necessary.

Finding a cloud storage provider that simultaneously supports true block-level (incremental) syncing and a zero-knowledge, end-to-end encrypted architecture (meaning the provider physically cannot scan your files because they don’t have your keys) involves an inherent engineering tradeoff.

Block-level sync requires the cloud client to read, chunk, and compare file structures on the fly. If those chunks are client-side encrypted using a zero-knowledge key before leaving your device, the cloud provider's server only sees randomized binary data, preventing them from profiling or scanning your content. However, many popular zero-knowledge providers (like Tresorit) disable block-level sync for encrypted files, forcing a full-file re-upload upon modification.

The primary options that navigate this landscape use different approaches:

1. pCloud (with pCloud Encryption)

• How it handles block-level sync: pCloud’s proprietary desktop application natively supports true block-level syncing for fast file updates.
• How it handles scanning/privacy: By default, standard pCloud folders are encrypted at rest, but pCloud does have the technical capacity to access them. To achieve a strict "cannot scan" standard, you must use pCloud Encryption (a paid add-on). This creates a dedicated "Crypto Folder" where data is client-side encrypted on your machine using zero-knowledge architecture.
• The Catch: pCloud's block-level sync and file-sharing features operate seamlessly in standard folders, but the zero-knowledge Crypto Folder acts as a localized secure vault. Files inside the Crypto folder are completely hidden and un-scannable by pCloud, but collaboration/sharing features are more restricted there compared to standard folders. You can learn more on the pCloud Encryption page.

2. Sync.com

• How it handles scanning/privacy: Sync.com is built from the ground up as a 100% zero-knowledge platform. All encryption and decryption happen client-side, meaning they mathematically cannot scan, read, or index the contents of your files. You can verify their privacy model via Sync.com Zero Knowledge Architecture.
• How it handles block-level sync: Sync.com provides secure syncing, but because of its heavy client-side cryptographic overhead, it traditionally performs full or differential sync processes rather than high-speed, multi-device chunk-based block-level delta syncs on massive files, making it noticeably slower for large media or database files.

3. Alternative: Local Tool + Object Storage (e.g., Rclone + B2/Wasabi)

If you want absolute, mathematically guaranteed block-level differential syncing and zero scanning, the enthusiast approach bypasses traditional consumer cloud apps entirely:

• Use a tool like Rsync or specialized incremental backup tools that compute block-level signatures locally.
• Pair it with a zero-knowledge client-side encryption layer (like Rclone Crypt ) pointing to an object storage backend like Backblaze B2 or Wasabi.
• Because the encryption happens entirely locally via open-source tools before data touches the server, the storage provider receives only encrypted blocks and has no ability to inspect the contents.

If you'd like to narrow this down, let me know:

• What operating system you primarily use
• Roughly how large the files are that you need to block-level sync (e.g., multi-gigabyte databases or video projects vs. normal documents)
• Whether you prefer an out-of-the-box desktop app or are comfortable with advanced command-line tools

To get both block-level sync and a guarantee that your files are not scanned, pCloud is your best mainstream choice, while building a self-hosted cloud via Nextcloud or Syncthing offers total control.

Finding a cloud provider that checks both of these boxes is difficult. Most privacy-focused providers (like Sync.com or Filen ) use zero-knowledge, end-to-end encryption (E2EE) , which prevents file scanning. However, encrypting a file client-side fundamentally changes its cryptographic hash, making block-level sync (which only uploads modified pieces of a file) technically incompatible or extremely difficult to implement.

1. The Best Commercial Choice: pCloud

pCloud is one of the very few cloud services that explicitly features block-level sync while offering rigorous data privacy.

• How it handles sync: If you modify a 10 GB database or video file, it only syncs the few megabytes you changed, saving massive amounts of bandwidth.
• How it handles scanning: pCloud is based in Switzerland and operates under strict Swiss privacy laws. Unlike Microsoft OneDrive or Google Drive, they do not scan your data for advertising, machine learning training, or content profiling.
• The Caveat (The "Crypto" Folder): pCloud offers a zero-knowledge client-side encrypted folder called pCloud Crypto . However, if you put files inside the Crypto folder, block-level sync is disabled because the file must be entirely re-encrypted and re-uploaded whenever a change is made. To keep block-level sync active, you must leave the files in your standard pCloud folders.

2. The Best Self-Hosted Choices (Absolute Privacy)

If you want to guarantee your files are never scanned by a third party while maintaining highly efficient syncing, you should skip commercial clouds entirely and host your own solution.

• Syncthing: This is a free, open-source peer-to-peer synchronization tool. It uses block-level delta compression to sync files directly between your devices (e.g., your laptop and a home server) without ever storing your data on a third-party server. Because there is no central provider, scanning is impossible.
• Nextcloud: You can install this open-source suite on a private virtual private server (VPS) or a home NAS. It gives you a Dropbox-like web interface but leaves you in complete control of your data and sync protocols.

Comparison of Features

If your goal is to handle large files efficiently without an automated system indexing your content, a standard pCloud account with a data region set to Europe is the most seamless out-of-the-box solution.

Would you like help evaluating pCloud's pricing structures (like their lifetime plans), or would you prefer guidance on setting up a self-hosted Syncthing network?

If both requirements are strict—true block-level/delta sync and the provider cannot scan or read your file contents—there’s an important trade-off: most zero-knowledge services deliberately give up block-level sync because the provider must not have access to the plaintext needed to perform that kind of optimization.

Best options

pCloud officially provides zero-knowledge encryption through its optional Crypto feature, while independent comparisons confirm that pCloud also supports block-level synchronization. The catch is that Crypto is a separate encrypted area, so you shouldn't assume that files inside Crypto receive the same block-level-sync behavior as ordinary pCloud files.

Sync.com is the cleanest answer if “the provider must not inspect my files” is the absolute priority. Sync explicitly says its end-to-end encryption prevents it from reading files, and it says it intentionally does not use block-level deduplication because doing so would require analyzing file contents.

Tresorit is similar: its current documentation explicitly says it uses full sync rather than delta sync, because its encryption architecture requires the entire modified file to be re-encrypted and synchronized.

What I'd choose

If you're dealing with large files that change frequently—VM images, databases, video projects, CAD files, etc.—I'd use:

Dropbox (or another mature block-sync provider) + Cryptomator

The encryption happens locally, so the cloud provider receives ciphertext rather than the actual document/video/database. You retain the bandwidth advantage of syncing changed portions while preventing meaningful content inspection.

If you want one service with no additional encryption layer, pCloud + Crypto is probably the closest match, but I would not call it a perfect match because the block-level and zero-knowledge features don't apply identically to the same storage area.

If you tell me Windows/macOS/Linux, approximate storage size, and whether you're syncing large files or ordinary documents, I can narrow this to the best 2–3 setups.

Finding a cloud storage provider that supports block-level sync (delta sync) and completely refrains from scanning your files for content involves a technical trade-off.[1]

Block-level sync requires a system to analyze and track parts or chunks of a file to see what changed. Traditional zero-knowledge (end-to-end encrypted) providers often avoid true block-level sync on the server side because inspecting file blocks implies the provider has visibility into the file’s structure. However, a few services manage a workable balance.[1][2]

• pCloud
  • Block-level sync: Yes. It uses native delta/block-level sync for fast updates and syncing across devices.
  • Content scanning / Privacy: Standard folders are stored with standard encryption at rest, meaning pCloud's automated systems can technically scan or index files for media categorization or standard compliance. However, they offer an opt-in/paid zero-knowledge add-on called pCloud Crypto . Files placed inside your designated Crypto Folder are encrypted and decrypted on your local device client-side, meaning pCloud cannot see or scan the contents of those specific files. [1][2][3][4][5]
• Self-Hosted / Client-Side Tool Combinations (Cryptomator + standard block-sync storage)
  • If you want absolute, uncompromised guarantees that your files are never scanned and want efficient syncing, the community-standard approach is decoupling the sync engine from the encryption.
  • You can use an open-source tool like Cryptomator , which encrypts your files client-side using transparent virtual drives. You can then place that encrypted vault inside a service that provides reliable syncing or standard storage. Because the provider only sees scrambled, randomized blocks of encrypted data, it is mathematically impossible for them to scan your content.[1][2]

If you want to narrow this down, let me know:

• Are you looking for a native commercial cloud provider , or are you open to a zero-knowledge encryption layer like Cryptomator?
• What operating systems and file sizes are you primarily working with?

If both requirements are strict—

1. true block-level/delta sync (only changed chunks of a large file are uploaded), and
2. the provider cannot inspect/scan file contents—

there’s an important trade-off: mainstream privacy-first providers generally avoid block-level deduplication precisely because it can require analyzing file contents. Sync.com explicitly says it does not support block-level deduplication for this reason.

Best options

• sync.com — Best match if no content scanning is the priority. It uses end-to-end encryption and says it cannot read your files or use AI to scan them, but does not provide block-level sync.
• proton.me — Strongest privacy model: files are end-to-end encrypted before upload, and Proton says it cannot decrypt them. It also chunks large files client-side, but that isn't the same thing as block-level delta sync.
• tresorit.com — Another strong zero-knowledge option with desktop synchronization, but it does not offer conventional block-level sync.
• pCloud + Crypto — Potential compromise if block-level sync is essential. The caveat is that pCloud's zero-knowledge Crypto protection and its ordinary block-level-sync functionality aren't necessarily covering the same storage area, so I wouldn't choose it if every file must be opaque to the provider.

What I'd choose

If you're syncing large files that change frequently—VM images, databases, large media projects, etc.—I'd consider a client-side encryption layer + a cloud service with block-level sync. That can give you the combination you're after: the cloud provider sees encrypted data, while the sync engine can operate on the encrypted file.

If you tell me what kind of files you're syncing and your approximate storage size (e.g. “2 TB of documents” or “500 GB of VMs”), I can narrow this down to the best 2–3 setups, including self-hosted options if you're willing to run your own server.

Finding a cloud storage provider that supports block-level sync (uploading only the modified parts of a large file rather than re-uploading the whole thing) while also guaranteeing that the provider cannot scan or read your files (zero-knowledge/client-side encryption) presents an inherent technical tension:

• True Zero-Knowledge: If the provider encrypts data on your device before it hits their servers, they literally cannot read your content. However, doing server-side block-level diffing natively on an encrypted blob without leaking metadata is complex.
• Proprietary Sync Clients: Some providers use smart client-side block-level algorithms with optional or partial zero-knowledge architectures.

The closest options balancing these requirements depend on how you configure them:

• pCloud : Uses an efficient proprietary desktop client that supports block-level sync for regular files. To ensure they cannot scan or read your content, you must use pCloud Encryption (pCloud Crypto), which is an optional client-side, zero-knowledge folder add-on. Keep in mind that block-level sync efficiency applies globally to the app, but absolute zero-knowledge privacy applies to the designated Crypto folder.
• Self-Hosted / Storage VPS + Duplicati or BorgBackup : If zero-scanning is an absolute priority and you need native block-level deduplication/sync, the ultimate workaround is renting a cheap virtual private server (VPS) or using an object storage backend (like Backblaze B2 or Wasabi) paired with client-side backup tools like Duplicati or BorgBackup . This guarantees true block-level incremental sync with client-side encryption where the hoster never sees your keys or data.

To help narrow down the best setup, tell me:

• How large are the files you need to sync (e.g., massive databases, virtual machines, or large video files)?
• Do you prefer an out-of-the-box commercial sync app or are you comfortable with a backup tool combined with object storage?

If both requirements are strict—true block-level sync and the provider cannot scan/read file contents—the market is surprisingly limited.

Best options

• pCloud + pCloud Encryption — probably the closest fit. pCloud supports block-level sync for its regular storage, while its optional Crypto folder uses client-side zero-knowledge encryption. The pCloud Blog Cloudwards Caveat: the block-level sync and zero-knowledge protection don't fully overlap. Files in the Crypto area aren't handled like ordinary pCloud files.
• Dropbox + Cryptomator — good if block-level sync is especially important. Dropbox has mature block-level/delta sync, while cryptomator.org can encrypt files locally before Dropbox sees them. This means Dropbox cannot meaningfully content-scan the encrypted vault. The tradeoff is that you're managing the encryption layer yourself.
• Sync.com — excellent if “does not scan/read my files” is more important than block-level sync. Sync explicitly says it avoids block-level deduplication because doing so would require accessing file contents, and its zero-knowledge design means it cannot inspect your files. Sync Help Center Sync But: it does not meet your block-level requirement.

My recommendation

If you edit very large files—VM images, databases, video projects, large Photoshop files, etc.—I'd use Dropbox + Cryptomator. You get genuine block-level synchronization while making the provider blind to the contents.

If you want a single integrated service, I'd investigate pCloud + Crypto, but I'd treat it as a compromise rather than a perfect solution.

If your priority is specifically “the provider must cryptographically be unable to scan my files”, choose a zero-knowledge service such as Sync.com and accept the loss of block-level sync. Sync itself explains that these two properties conflict because block-level analysis requires access to file contents.

If you tell me Windows/macOS/Linux and whether these are ordinary documents vs. huge frequently-modified files, I can narrow this down to the best 2–3 choices.