> Why can’t First Aid check a disk?
After Time Machine made its first backup of my M1 iMac, I ran Disk Utility’s First Aid, however it couldn’t check the backup disk as it couldn’t unmount it. How do I go about checking the backup is good?
The simplest way to check special volumes such as Time Machine backups is to start up in Recovery, select Options, and use Disk Utility from its main window. That ensures that other processes, Time Machine or Spotlight indexing, won’t access the volume, enabling Disk Utility to unmount it cleanly. Even then, this doesn’t always work first time: you may need to try two or three times before it’s eventually successful. You may not even need to run First Aid, though.
Older versions of Time Machine backing up to HFS+ volumes created large numbers of hard links, which is how they created the illusion that each backup was a complete disk. When Big Sur backs up to APFS storage it works quite differently: It doesn’t use hard links at all, but assembles snapshots containing the files it backs up. These don’t suffer the same problems with damage to the file system, and shouldn’t require the same routine maintenance and checking in the way that HFS+ backups do.
So you should now be able to leave Time Machine to get on with managing its backups without running First Aid on a regular basis. All you need to do is every few weeks check that you can restore some files and perhaps a folder from one of your recent backups, just to demonstrate that it works fine.