SouthernEnglander wrote:
My M1 iMac stopped making backups with Time Machine at mid-day on 13 January 26 having worked flawlessly for the previous year. I can see, read and restore from the backups I have but all new backup attempts fail with the message "backup can't be made because the backup disk was previously encrypted but is now unencrypted". When I look at the drive with the disk utility it reports the drive as encrypted. When I try and enter the Time Machine settings the normal options are not present, all I see is an error message about encryption.
This is a problem because it sounds like your Time Machine backup drive is basically inoperable. It won't back up anymore nor can you restore from the Time Machine interface. Even though you can see files on that drive, can you reliably restore from it? Could you restore your entire internal drive from it if you needed to?
Noting that a new version of Tahoe beta MacOS was released January 12, 2026 ... are you using the beta? Your problem started January 13. Maybe that is just a coincidence, and irrelevant if you are using the official release of 24.2. Problems with beta software are off limits for Apple Discussions, however. There may be separate Developer forums for that, I'm not sure.
I agree with den.thed that a multiple backup, multiple software diverse backup strategy is essential. Imagine that a power surge ruins both your internal drive AND the Time Machine backup drive that is in progress at that moment. I would have a separate "clone" type backup that is disconnected when not being updated. A third backup, preferably stored off site or in the cloud, should be considered.
For now, you should get a new backup drive and get everything backed up right away because what you have now is questionable and not reliable. I suspect something is amiss (hardware possibly) with the backup drive. If you are using the drive manufacturer software/firmware that may conflict with the MacOS and in that case I would erase the backup drive, format it with Apple's Disk Utility as APFS/GUID, and then start over with a new backup.
