Time Machine Backups - refuses to replace old backups

A question and a few comments.

What operating system is currently installed on your Mac?

It recently became full and TM refuses to backup on it anymore.

The theory with Time Machine is that once the disk is full, that Time Machine will start to delete the oldest backups to make room for the newest backups. What Apple does not tell us though......or it is in the very small print......is that when the disk is full and the next backup is larger than usual......as might occur after an update, a number of new photos or other data.......Time Machine cannot delete enough of the old backups to make room for the newer larger than normal backup, and everything comes to a halt.

I don't think the snapshots are the real issue here........because snapshots are stored on the Mac......not on the external drive. But, it would not hurt to delete all the snapshots on the Mac just to help clear the clutter.

Open System Preferences and then open Time Machine

Remove the check mark from the box next to Back Up Automatically

Wait another 15-20 minutes. Longer will not hurt

Then, enter the check mark back in the box next to Back Up Automatically

That will delete all the old snapshots and new snapshots will begin. Technically, a snapshot does not take place unless the regular back up drive is not available, so you probably won't need to delete the snapshots on the Mac again for some time.

The next task is to decide what to do about the external drive that is full. Would it be the end of the world if you erased the drive and started over again with a new complete backup? Few users ever need to go back weeks, months and even years to pick up a file. You'll have a nice history of new backups in month or two.

If you cannot bear to think about erasing the backup drive, the only other option becomes adding a new drive and keeping the old drive in a safe place in case you need to go back and pick up a file. After a few months, you might decide that you don't really need all the old backups, so you could erase the drive and use it as a spare......or....set up dual Time Machine backups......or use Carbon Copy Cloner to keep a "clone" of your Mac on the spare drive.

Your turn to comment now.

Nice work. It would not hurt to run Disk Utility - First Aid on Macintosh HD to correct any errors that might be slowing things down a bit.

It's good that you are using an external drive connected directly to the Mac because the drive can be formatted in APFS, which is the same way that your Mac's hard drive is formatted. If you were backing up to a network drive......like a Time Capsule......Time Machine must back up to the drive formatted as Mac OS Extended (Journaled), so data on your Mac, formatted APFS has to back up the Time Capsule.....formatted as HFS+. Gosh, I wonder what could go wrong with this type of setup.

Not sure what the capacity of your external drive might be, but the general recommendation is that it should be 2-3 times the size of drive on your Mac to allow for a long history of backups (even though most of us never go back more than a few days to pick up file that we might have deleted by mistake).

Good luck!