So I was just doing my weekly Time Machine backup (besides my regular TM, I have a 18TB disk for weekly TM backups), and decided to check the drive with DU. The backup seemed to go OK, and the device and the Volume check out OK (with 37 Snapshots), but the container check is telling me [error: container /dev/rdisk12 is mounted. / Storage system check exit code is 65. / Storage system verify or repair failed. : (-69716)]. Is there a way to save the TM backups and do I need to reformat the drive all over? Can I copy the TM Snapshots via the finder, reformat, and then copy them back?
Use Disk Utility to attempt a repair using First Aid on 1) TM Volume, 2) the APFS container, and 3) the drive. That will take a long time on a large HDD with 37 snapshots.
If that fails you will need to discover what is readable on the disk and save what you can.
Assuming they are readable, you can save snapshots (via Finder) to preserve important data. But each snapshot will take up its own disk space, so it is not practical to save every snapshot, reformat and copy them back. Each of the 37 snapshots will take up their own disk space. The time that would take also makes it impractical.
Also, be aware that a Finder copied snapshot is no longer a snapshot - it now a bunch of folder and files without the snapshot structure around them. Apart from the disk space issue, it can no longer be used by Migration Assistant for a system recovery.
More practical (in terms of time and space) is to preserve key items from a few selected snapshots.
Before reusing that disk for Time Machine do think about whether you can trust it for future backups.
Once a backup disk goes bad, I wouldn’t trust it for anything. I would buy a new disk for your weekly TM backups, and set the old one aside, not to be touched unless you find you need to access a file version that isn’t on your standard TM disk.
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