macOS 27.0.1: Time Machine reports "Backup Failed" daily for an intentionally offline destination
Setup: two Time Machine destinations. An SMB share on a NAS (always connected, hourly backups succeed) and an external disk that I connect only occasionally and otherwise store offsite.
Since macOS 27.0.1, Time Machine shows "Backup Failed…" in the menu bar and in System Settings > Time Machine every day for the offline external disk. The info button says: "The backup disk could not be found. Make sure the backup disk is connected or select a different backup disk." This did not happen on earlier macOS versions, where the disconnected disk was silently skipped.
The backupd log shows the scheduler rotating to the external disk on each run, logging "Destination … could not be found", and then falling back to the NAS, which backs up successfully. Nothing is actually wrong with either backup.
I've filed this as FB25485292. If you use an offsite/rotated Time Machine disk alongside an always-on destination and see the same warning, please file your own report and mention this ID.