Do Time Machine and Carbon Copy Cloner verify your backups?

By Ting Tze Chuen · Updated October 7, 2026

Does Time Machine verify backups?

Not in the way most people assume. Time Machine works out which files have changed since the last backup, mostly from the file system's change log (FSEvents), as Howard Oakley's analysis of Time Machine describes. It copies those files and moves on. It doesn't read each copy back from the backup disk to confirm it matches.

There are three partial exceptions, and which one applies depends on where your backup lives:

• Network backups (a Mac or NAS shared over the network): Apple's guide to verifying your backup disk says Time Machine periodically checks these automatically, and you can start a check yourself with Verify Backups.
• HFS+ (Mac OS Extended) backup disks: since OS X 10.11, Time Machine records a checksum for each file it copies, which tmutil verifychecksums can check later.
• APFS backup disks, the format Time Machine uses for new local backup disks today: neither Verify Backups nor tmutil verifychecksums checks file contents, according to Oakley's tests on macOS Sonoma.

So if your Time Machine backup is on a USB or Thunderbolt drive, you're probably relying on the drive to report write errors. Silent corruption, where a drive accepts data but stores it wrongly, won't be noticed until you try to restore.

Copying by date and size vs comparing checksums

Backup tools decide what to copy in one of two ways, and "verify" means something different in each.

Date and size (or a change log). The tool compares each file's size and modification date, or asks macOS what changed. If they match the backup, the file is skipped. This is fast, which is why Time Machine, Carbon Copy Cloner's normal mode, and rsync by default all work this way. The catch: a file that hasn't changed on your Mac is never read again, so if its backup copy quietly rots, nothing notices.

Checksums. The tool reads the full contents of a file and calculates a fingerprint, such as SHA-256. If one bit differs, the fingerprint changes. Comparing the fingerprint of the original with the fingerprint of the copy proves the contents match. It's slower, because every byte has to be read on both sides.

"Verify" can therefore mean anything from "the drive didn't report an error" to "every byte was read back and matched". When a backup app says it verifies, it's worth finding out which.

How to run Verify Backups in Time Machine

This applies when you back up to a network disk. From Apple's instructions:

1. Click the Time Machine icon in the menu bar. If it isn't there, go to Apple menu > System Settings > Control Center, find Time Machine, and choose Show in Menu Bar.
2. Hold down the Option key. The menu changes to show extra commands.
3. Choose Verify Backups.

If the command is dimmed, your backup is most likely on a directly connected disk. Oakley found it either unavailable there or simply starting an ordinary backup, with no sign of a verification.

What does tmutil verifychecksums check?

tmutil is Time Machine's command-line tool, built into macOS. According to its manual page (man tmutil), verifychecksums recalculates checksums of files inside a backup and compares them with the checksums Time Machine recorded when it made the backup. Matching files produce no output. A ! marks a file whose checksum no longer matches; a ? marks a file whose recorded checksum is invalid.

Checksums are only recorded for files copied by OS X 10.11 or later, and in practice only on HFS+ backup disks. To check your latest backup, open Terminal and run:

sudo tmutil verifychecksums "$(sudo tmutil latestbackup)"

Enter your Mac's password when asked. If Terminal reports "Operation not permitted", give it Full Disk Access in System Settings > Privacy & Security > Full Disk Access, then quit and reopen it.

What about tmutil compare?

sudo tmutil compare lists differences between your Mac and its latest backup. By default it compares metadata only: sizes, modification dates, permissions, owners and extended attributes. Adding -d also compares file contents, but because any option replaces the default set, use -s -t -d to check size, date and contents together. It's useful for spotting what changed, but anything you've edited since the last backup shows up as a difference, so it's hard to read as a health check.

How to spot-check a Time Machine backup yourself

For an APFS backup disk, the practical option is to compare a folder that hasn't changed since the backup, such as last year's photos, with its copy in the backup:

1. Eject the backup disk and reconnect it, so the check reads from the disk rather than from memory.
2. Open the backup disk in Finder and browse into the most recent dated backup until you reach the folder you want to check.
3. In Terminal, type diff -rq followed by a space, drag the original folder from your Mac into the window, type a space, then drag in the matching folder from the backup.
4. Press Return. No output means every file matches. Otherwise diff lists files that differ or exist on only one side.

The finished command looks like this:

diff -rq ~/Pictures/2025 "/path/to/backup/…/Pictures/2025"

The real path to the backup copy is long and includes the backup date, which is why dragging it in is easier than typing it. Our guide to verifying a file copy on Mac covers checksum lists you can save and re-run later.

Does Carbon Copy Cloner verify backups?

Yes, if you turn it on. Bombich's guide to how to verify a backup describes three options. CCC normally uses file size and modification date to decide what to copy, so none of them run unless you enable them.

Re-verify files that were copied

CCC calculates a checksum as it writes each file. With this setting on, it reads back the files copied during that run at the end of the task and confirms they match. Bombich suggests it especially when moving data to a new device or before deleting the originals.

1. Click Advanced Settings at the bottom of the CCC window.
2. Select the Postflight tab.
3. Check Re-verify files that were copied.

Backup Health Check

Formerly called "Find and replace corrupted files", this setting calculates a checksum of every file on the source and the destination. If a backup copy differs and the source file is fully readable, CCC copies it again. If the source file can't be read, CCC leaves the existing backup in place and reports an error. Because it re-reads everything, Bombich recommends limiting it to weekly or monthly runs.

1. Click Advanced Settings.
2. Select the Performance & Analysis tab.
3. Check Backup Health Check, then choose an interval from the popup menu next to it.

To run one check without changing the task, hold Control and click Start.

Verify files copied by this task

CCC keeps a record of the size, modification date and checksum of each file it copies. Click the Source or Destination selector and choose Verify files copied by this task to check either side against that record without running a backup.

Is CCC's Compare button a verification?

No. Bombich's Compare documentation says it highlights folder size differences between source and destination and is "not a byte-by-byte verification of files". It's handy for answering "why are these sizes different?", not for proving the backup is intact.

Which backup tools verify after copying?

Where VerifiedCopy fits, and where it doesn't

VerifiedCopy is not a backup scheduler. It doesn't run on a timer, keep old versions, or back up your whole Mac, so keep using Time Machine or CCC for that.

It's for the one-off copies those tools aren't built around: offloading a camera card, archiving a finished project to two drives, or moving a photo library to a new disk before you delete the original. It copies to up to three destinations at once. Each chunk is written, flushed to the drive, read back and its SHA-256 compared with the source; a chunk that doesn't match is rewritten and checked again. Its Verify & Repair mode can also scan an existing backup against a known-good source and rewrite only the damaged parts.