Unable to backup using time machine

Was it working without issues before you updated to 15.0.1? There is a defect in the last three versions of Sequoia: 15.01, 15.1 and 15.1.1 that 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. 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.

Apple has no solution, nor it has offered any solution or workaround. They know this from mid-October but nothing. Do not bother to update to 15.1 or 15.1.1 because neither addresses the issue. The support team at Apple will insist that you upgrade and call them back if it does not work. It won’t.

I resolved the issue after working with an Apple support person. I have an MBP M1 Pro with MacOS 15.1.1. and I had failures with TM both on NAS and hard wired backups. My workaround, as per the boards, was to to turn off the MacOS Firewall to let the backups work to completion.

However, I also have a cyber security software which includes a firewall of its own. There is a conflict between its firewall and the MacOS one starting with Sequoia 15.x. After I turned off the security software firewall and left the Mac OS one active, all TM backups work without error. The 3rd party cyber security software firewalls need to be updated for Sequoia.