Can't back up to external device since Sequoia arrived.

Was it working without issues before you updated to 15? 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.

How to write an effective question.