I have an awful lot of photos - about 2.5TB. I keep more recent shots (principally from this year and last) on the internal 2TB SSD of my Studio and older shots on an external 2TB SSD, which is permanently connected. Both are backed up by TM. To make space on the internal drive for a few hundred more megabytes of shots I took last week, I moved some older folders to the external drive.
TM now has those folders backed up twice, once for each disk, and so is reporting that its backup drive is full and backups are failing. On older systems, I could just have deleted all the backups of the internal drive folders, but that’s not possible now (Sonoma).
Is there anything I can do apart from reformatting my TM backup drive (discarding all backups) and starting again?
It’s surprising that the old backups are not being deleted automatically from your backup drive by Time Machine.
If the SSD is formatted as APFS, the backups of folders residing on the internal disk will remain until those backups containing those folders age out. You’re right that you can no longer delete individual files out of a backup - especially if the Time Machine media is formatted as APFS.
You don’t say how big your Time Machine backup disk is and if its formatted APFS or HFS+. One option if you’ve got an APFS formatted Time Machine disk (and you don’t want to reformat the drive and lose everything) is to open the Time Machine drive in the Finder and manually delete old backups until you have enough storage to complete the new backup. At the top level of the Time Machine drive, right click on the oldest backup and select “Delete Immediately…”. You may need to wait a bit before the APFS snapshot on the backup disk is actually deleted to see how much space you get back at each deletion.
Time Machine is best used for system recovery, not long term data archival. Expecting to be able to recover any file that you needed to save forever from a Time Machine backup you took years ago is not practical. That’s where the discussion of archival vs. backup comes into play.
My philosophy on Time Machine backups is to use a fairly large drive and when it gets full retire it and start with a new one. The old drive gets labeled with start and end date and stored in my fireproof file.