Time Machine stalling while Mac is alseep
Time won't finish a back-up when my computer goes to sleep. There used to be a Nap setting but no longer available.
Mac Studio, macOS 15.1
This is not a third party issue. This is an Apple defect that has several variations and workarounds found by the boards. Apple has not offered yet any credible solution or explanation. Apple support and enginnering know this from mid-October but nothing has been offered as a solution. The support team at Apple will insist that you upgrade and call them back if it does not work. It won’t.
The defect in the last three versions (at least) of Sequoia: 15.0.1, 15.1 and 15.1.1 makes TM backups fail with either “Files not available” when the backup is about to end, or ”Hard disk was disconnected” fairly quickly after starting the back-up process for NAS locations and local (hardwired) hard disks.
Check the threads:
What causes TimeMachine backups to fail on NAS devices?
Time Machine did not finish backing up because some files were unavailable. Backups will resume when your Mac is unlocked.
Since update to MacOS 15.0.1 Time Machine backups do not work
I have 15.0.1 and the workaround I found in the boards was to turn off the MacOS Firewall (System Settings > Network > Firewall) and the TM backups complete without errors. It appears that it does not work for everyone but it is worth a try.