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A coding agent helped me audit my NAS for duplicate files, and I found about 2TB of copies I had been backing up twice. That is my experience, not an independently verified result or a promise of savings. A duplicate report can show matching files in the locations it scanned; it cannot, by itself, show what a backup provider charged for or retained.
The useful lesson is less dramatic but more reliable: use an agent to help inspect and organize the audit, treat its findings as candidates, and verify both the files and any claimed billing impact before deleting anything.
What the 2TB figure does—and does not—mean
In my audit, the duplicate files added up to about 2TB of storage that I was backing up twice. I have not independently verified that scan, the total, the backup provider’s accounting, or a reduction in my bill. The figure describes storage in my anecdote; it is not a dollar amount or a typical result.
A file-level duplicate finder reports matches within the folders, file types, and matching rules it examined. It does not know from that report alone whether a backup service stored both copies, how long it retained them, or how it calculated a charge. Freed space on a NAS is not proof of a smaller backup invoice.
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#1 Best Overall
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What a coding agent can help with
A coding agent can assist with the audit work: helping define scan scope, interpreting a report, grouping candidate files by path or size, or preparing a reviewable cleanup plan. That assistance is not the same as a verified scan. The tool doing the file matching, its configuration, and the files it could access determine what the report means.
For example, rmlint 2.11.0-rc.0 documentation, updated September 18, 2026, describes duplicate-file and directory detection, hash-based matching, and an option for byte-by-byte comparison. Those methods can identify identical files; they do not establish that two differently edited documents or similar-looking images are duplicates. Any account of an audit should say which method and locations were used rather than suggesting that an agent examined an entire NAS by default.
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How to audit a NAS without risking the wrong files
- Define the scope. Record the NAS model and operating-system version, the shared folders and exclusions, file categories, any minimum-size threshold, and whether the scanner compared hashes or bytes. A report only describes what was included in that scan.
- Make sure you can recover. Keep a current, restorable backup before substantial reorganization or deletion. The rmlint safety guidance recommends current backups before major changes and warns that duplicate finders can have false positives or more serious bugs.
- Start with a report or dry run. Inspect representative groups, including their full paths, sizes, and timestamps. Decide which copy to keep based on how you organize files and what applications may depend on them; a matching-content result cannot make that choice for you.
- Quarantine before deleting. Move candidates to a holding area first, then check that expected applications still work and that the retained files are available. rmlint discusses holding areas and filesystem-dependent reflinks; do not assume reflinks or hardlinks are safe or portable across filesystems or backup and restore workflows.
- Check the backup and the bill separately. Confirm that the cleaned-up data remains restorable. For a cost claim, consult the provider’s own usage accounting and billing rules rather than treating NAS space reclaimed as a charge avoided.
File duplicates are not the same as backup deduplication
These terms describe different layers. A duplicate-file tool looks for repeated files or directories in the locations it scans. A backup product may instead store incremental changes or deduplicate data blocks within its own backup system. Neither behavior can be assumed for every NAS or service.
| Approach | What it describes | What it does not establish |
|---|---|---|
| File-level duplicate detection | Matching files or directories in the scan scope, using the selected method. rmlint documents hash-based detection and byte-by-byte comparison. | Whether a backup provider billed twice, retained both copies, or will lower a charge after cleanup. |
| NAS analysis report | Synology Storage Analyzer advertises duplicate-file detection, CSV export, custom analysis criteria, and default categories including archives, pictures, videos, audio, documents, web pages, disk images, and executable files. | Whether another NAS offers the same feature or whether the provider’s backup storage is deduplicated. |
| Backup-system deduplication or incremental backup | Product-specific handling of changed data or repeated blocks in a backup workflow. | That an unrelated file-level scan will reduce the backup size or invoice. |
What Synology’s tools document
Storage Analyzer
Synology’s Storage Analyzer specification lists duplicate-file detection, CSV report export, custom criteria, and the categories shown above. It says each report finds a maximum of 5,000 duplicate files. The specification page lists DSM 7.2 and 7.3; check that your installed DSM version and NAS model support the feature before following any product-specific instructions.
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Backup and snapshot products
Synology’s backup guidance describes Hyper Backup as supporting block-level incremental backup and cross-file deduplication, and Snapshot Replication as replicating changed data blocks. These are behaviors of the named Synology products, not evidence that a separate duplicate-file scan changes what a cloud provider charges.
Synology’s Active Backup for Business white paper describes a separate, product-specific design: destination-side inline deduplication, global deduplication across tasks within one shared folder, 4 KB chunks fingerprinted with SHA-256, and changed-block tracking for incremental backups. Its examples and measurements belong to that product and white-paper context, not to a general NAS or personal cloud-backup estimate. Likewise, Synology’s snapshot and storage-efficiency guidance notes that snapshots can affect whether deduplication or compression tasks can process data; operational details should be checked against the exact NAS, DSM version, and snapshot setup.
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What to document before reporting a result
If you describe an audit as evidence of duplicated backup costs, make the chain of evidence visible. Report the scan scope and matching method, the duplicate count and total bytes, what was moved or removed and which copies were retained, and the provider’s accounting or invoice that supports the cost claim. Without provider-side evidence, describe the result as duplicate NAS data identified—not as a verified billing reduction.
Quick Recap
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