A check reports changed or damaged files

What happened

A backup check, a trial restore, a content audit or a disc check found files that are missing or that differ from the recorded checksums or from the seal. The cause may be a failing drive, an interrupted write or a bad USB cable, but also deliberate tampering, for example by malware.

What to do

Do not overwrite anything. Do not restore damaged files over the originals.
Check again after reconnecting the drive, ideally with another cable and to another USB port. If the errors are different every time or disappear, the culprit is often the cable, the port or the drive enclosure.
Check whether the originals are intact. If the errors were reported by “Check backup” and the files on the computer are fine, run the backup: the program writes the damaged files again from the originals. The preview of changes shows them as files damaged in the backup. Then check the backup again. If the errors come back or the drive is failing, make a new backup in a new folder on another, healthy drive (see “Creating a backup without the wizard”). Do not delete the old backup until the new one has been checked.
If the originals are gone, recover the files from another source: from an archive disc (see “Restoring files from a disc”), from the off-site backup (see “Restoring from the off-site backup”) or from an older version (see “Finding a file in the backup and browsing its versions”). An older version helps only if the file changed between versions: an unchanged file is shared by all versions on the drive, so it is damaged in every one of them.
If the audit result is “TAMPERED WITH or damaged”, or there are many errors at once, check the computer with an antivirus program, and restore files from the version pointed to by “Last untouched” (see “Checking that nobody has tampered with files in the backup”).

Drive health

Windows can check the drive: in File Explorer, right-click the drive, choose “Properties”, the “Tools” tab and “Check”. If the drive has bad sectors or makes unusual noises, do not repair it; make a new backup on another drive as soon as possible. Discs have their own recovery data (see “Checking a disc”).

If something goes wrong

Every encrypted file reports “Verification failed: wrong password or a damaged file.”: this is usually a wrong password, not damage (see “I don't remember the backup or disc password”).