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

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.

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.

• 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.
• 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.

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.

There is a fundamental cryptographic tradeoff in cloud storage: True zero-knowledge (end-to-end encrypted) privacy makes block-level sync technically impossible.

To perform block-level sync (analyzing and uploading only the changed chunks/bytes of a modified file), a cloud provider's server or client must be able to inspect and parse the internal blocks or structures of that file . If the provider has the keys to inspect or de-duplicate blocks, they do not have zero-knowledge privacy, meaning they can—and often do—scan files . Conversely, if a service is truly zero-knowledge (encrypted client-side so the provider cannot read or scan your content), any tiny change to a file alters the cryptographic hash of the entire file, requiring the whole file to be re-encrypted and re-uploaded.

The options below outline how popular providers navigate this trade-off:

• pCloud (with pCloud Crypto) : Supports true block-level file synchronization for regular folders . However, default pCloud storage is not zero-knowledge, and they do scan unencrypted files for copyright/compliance via automated checksums . To get zero-knowledge privacy, you must use their paid pCloud Crypto folder , which acts as a client-side encrypted vault where they cannot see or scan your files (though block-level sync behavior changes inside the crypto vault). You can explore plans on the pCloud Pricing Page.
• Sync.com : Offers zero-knowledge encryption by default across your entire account, meaning they cannot see, read, or scan your files . Because of this strict privacy commitment, Sync.com does not use block-level sync (modified files re-upload). If absolute zero content scanning and zero-knowledge privacy is your priority, you can check out details on the Sync.com Security Overview.
• Tresorit : Known for enterprise-grade, zero-knowledge end-to-end encryption where the provider has zero visibility into your content . Like Sync.com, Tresorit does not support block-level sync because their client-side encryption architecture requires entire modified files to be re-encrypted and synced . You can read more via the Tresorit Features Page.

To help narrow down the best compromise for your workflow, let me know:

• Is your primary priority absolute privacy (zero-knowledge) or saving bandwidth/speed (block-level sync)?
• How large are the files you expect to be modifying frequently?

You’re looking for a fairly unusual combination: true block-level/delta sync plus a provider that cannot inspect or scan file contents.

Based on current documentation, the choices are:

My recommendation: pCloud, with an important caveat

pCloud offers block-level synchronization, while its optional pCloud Encryption/Crypto feature uses zero-knowledge encryption: encryption happens on your device and pCloud says it cannot access the encrypted data.

The catch is that the block-level-sync and zero-knowledge properties don't necessarily apply to the same storage area. If every file must be inaccessible to the provider, pCloud isn't a clean answer.

If “no scanning” is non-negotiable

I'd choose Sync.com or Tresorit and give up block-level sync. Sync.com explicitly says it doesn't perform block-level deduplication because doing so would require accessing/scanning file contents, and it deliberately avoids this to preserve zero-knowledge privacy. help.sync.com Tresorit likewise uses end-to-end encryption for synced cloud content.

Bottom line: If you mean strictly “the provider must never be able to inspect the contents, while modified portions of large files are synced rather than re-uploading the whole file,” I don't see a mainstream service that cleanly satisfies both requirements today. The underlying tension is that traditional block-level sync generally needs access to file/chunk information that zero-knowledge encryption intentionally hides.

If you tell me whether you're mainly syncing large files (VMs, video, CAD, databases) or ordinary documents, I can suggest the best architecture—including a client-side encryption setup that preserves block-level sync.

Your requirements are a difficult combination:

1. Block-level sync (only changed parts of a file are uploaded)
2. No content scanning (ideally the provider cannot read your files at all)

These two features often conflict because true zero-knowledge encryption prevents the provider from seeing enough of the file structure to efficiently perform server-side delta comparisons.

Here are the closest options:

Best practical choices:

• If block-level sync is mandatory → choose pCloud (using Crypto for sensitive files) or accept Dropbox-style privacy tradeoffs.
• If no scanning/provider access is mandatory → choose Sync.com, Proton Drive, or Tresorit.
• If you need both as strictly as possible → consider a hybrid:
  • Use a block-sync service for performance.
  • Encrypt sensitive folders locally with a tool such as Cryptomator before syncing.

So the closest single-service match is probably pCloud + Crypto, but there is not currently a mainstream service that provides full zero-knowledge encryption for all files plus true Dropbox-style block-level sync.

pCloud is the only mainstream cloud storage provider that explicitly meets both criteria.

Finding a service that does both is highly difficult because true zero-knowledge encryption usually breaks block-level sync . Most strict, non-scanning privacy clouds (like Sync.com or Tresorit) require you to re-upload an entire file if you change a single byte, because encrypting a file changes the blocks completely.

The Top Recommendations

• pCloud : This is your best native choice. By default, pCloud uses a standard server-side configuration that supports true block-level sync (delta sync) , meaning it only uploads changed portions of files to save massive amounts of bandwidth. Furthermore, they operate out of strict Swiss and European privacy jurisdictions and do not scan your data unless you are publicly sharing illegal content.
  • The Catch: If you purchase their optional zero-knowledge "pCloud Crypto" folder, you will lose block-level sync functionality inside that specific folder.
• The "Bring Your Own Cloud" Alternative (Cryptomator / Rclone) : If you want 100% guarantee against content scanning via local encryption, but still want block-level performance, you can use a standard cloud provider (like Dropbox or OneDrive, which have excellent block-level sync) paired with open-source tools. Using software like Rclone or Cryptomator lets you slice and encrypt files into small, obfuscated blocks locally on your computer before they ever reach the cloud. The cloud provider only sees unreadable chunks of data and cannot scan them.

Direct Comparison

To help narrow down the best setup for you, let me know:

• What operating system (Windows, macOS, Linux) do you use?
• What types of files (large databases, video projects, text documents) are you syncing?
• Are you looking for a free tier or a paid plan?

Finding a mainstream cloud storage provider that offers block-level sync and guarantees they do not scan your files for content is an architectural challenge.

True block-level sync requires the cloud provider's client and server to inspect and index parts of files to reconcile differences. Conversely, a strict zero-knowledge (end-to-end encrypted) service means the provider only sees scrambled ciphertext on their servers, making content scanning structurally impossible—but this heavy encryption usually prevents true block-level sync out of the box (services like Sync.com or Tresorit use file-level sync instead).

To achieve both requirements, you have two primary paths: a commercial hybrid approach or a self-managed backup tool combined with object storage.

• pCloud (with pCloud Crypto) : Offers native block-level file synchronization for fast incremental updates. By default, standard pCloud storage is server-side encrypted (meaning they technically could scan data, though they have privacy policies against it). However, you can add pCloud Crypto , which creates a client-side, zero-knowledge encrypted "Crypto Folder". Files placed inside this folder are locked down so that pCloud has zero knowledge of the contents, though block-level syncing applies efficiently across your account. You can review their tiers on the pCloud Plans & Pricing page.
• Restic or BorgBackup + Any Object Storage (S3, Backblaze B2, Wasabi) : If you want an ironclad guarantee that no one scans your files, use a backup client that performs content-defined chunking (block-level deduplication) locally on your machine, encrypts the blocks via zero-knowledge keys, and uploads only the diffs to a raw cloud bucket.
  • Restic splits files into variable-sized blocks locally, encrypts them, and handles incremental changes seamlessly.
  • You can pipe Restic output directly into a provider like Backblaze B2 Cloud Storage or an S3-compatible host. The provider only ever sees encrypted, meaningless data chunks and lacks the ability (or reason) to scan your content.

To help narrow down the best setup, tell me:

• Are you looking for a ready-to-use desktop sync app (like Dropbox style) or comfortable with a command-line / utility tool?
• What kind of files are you syncing (large databases/VM images, or standard documents/media)?

That combination is unusually hard to find. True block-level/delta sync and strong privacy often conflict because the service needs to determine which parts of a file changed.

Best options

Sync.com explicitly says it avoids block-level deduplication because doing so would require accessing/scanning portions of files, and its zero-knowledge design prevents it from inspecting them.

If your requirements are absolute — “only changed blocks transfer” AND “the storage provider cannot read/scan the contents” — I'd look at a self-hosted solution rather than a mainstream cloud provider. VaultDrift, for example, currently advertises client-side AES-256-GCM encryption, zero-knowledge architecture, and content-defined-chunking delta sync. vaultdrift.com I'd treat it as an emerging/self-hosted project rather than an established cloud service, though.

If you tell me what kinds of files you're syncing (e.g. VMs, photos, video, databases, ordinary documents) and whether self-hosting is acceptable, I can narrow this to the best 2–3 choices.