I've used Time Machine for years with a cheap HD hanging off an old Airport Extreme, until today, incidentally.

MacOS had started warning that this approach won't be supported in the future. After upgrading to Tahoe, Time Machine kept saying backup failed, no matter what I did, despite the fact it should still work. Oh well, I'll just delete the old backup and create a new one.

I delete the old backup, click "Add Backup Disk...", select the backup disk, and get blocked with "[Drive] can only be used if it contains existing Time Machine backups for this Mac." It did! You broke them!

UGH.

I thought I'd get another year out of it. Apple in their wisdom has decided otherwise. Now I have no historical or ongoing backups.

Any recs on what to use instead?