Time Machine reports backup disk full, not deleting old backups

Yes, the accepted proposal is that TM should delete older backups from the external drive to make room for newer backups. However, the reality is that this does not always happen. There are several reasons for this that I won't get into, but we see this complaint here often.

Your options are: 1) erase the backup drive and start a fresh backup set, or 2) set this backup drive aside for the time being and start a new backup set on another external drive.

There is nothing that the user can do to free up space on the currently full drive that doesn't risk corrupting the entire backup set.

Back up your Mac with Time Machine - Apple Support

If you can't back up or restore your Mac using Time Machine - Apple Support

Time Machine has a predefined schedule of what it can remove, and the remaining storage after that schedule is executed may not leave sufficient room to complete the current backup storage demand. I suspect that this is what you are up against now.

As others have suggested, I would set that current TM drive aside, label it with the last operating system backed up to it, and the date/time. Start another Time Machine 4 TB drive formatted as APFS.

You haven't mentioned it but if you have a Virtual Machine guest stored on your 2TB internal drive, I would exclude that from Time Machine backups because every time you use that guest, you impart change and Time Machine would back it up too. Same for mounted drives that you do not want included.

Khalifa94 wrote:

Time Machine is no longer backing up and I get this pop-up from time to time: 'Time Machine could not back up because the disk is full.' My Mac HD (running Tahoe 26.6.2) is 2TB and the EHD that TM is on is 4TB. It says that 3.9 TB have already been used. Isn't TM supposed to delete old backups so as not to have to continually increase storage? What am I missing here? TIA!

yes suppose to and often does, however there are reports like yours.

To get past your issue, your best bet is start anew.

If you value your user data

3-2-1 Backup Strategy: three copies of your data, two different methods, and one offsite.

More than one device, more than one backup methodology— and backup regularly.

—Back up your Mac https://support.apple.com/en-us/HT201250

Erase and reformat a storage device in Disk Utility on Mac

https://support.apple.com/guide/disk-utility/erase-and-reformat-a-storage-device-dskutl14079/22.7/mac/26

Thank you both, D.I. Johnson and leroydouglas. I erased my HD with Disk Utility and Time Machine is now up and running again. With only 3 hours to go. Appreciate your help!

thank you. Somehow I missed your message and went ahead and erased my HD and am attempting to backup again now. If I get the same message I will come back to this and ask more about the Virtual Machine guest, because I have no idea if I have that or not.

You're welcome.

Happy to know your backup is running again. 👍🏽

If you have not installed any Virtual Machine (Parallels, VirtualBox, VirtualBuddy, UTM, VMware, etc.) software then you needn't worry about a virtual machine guest (e.g. Linux, Windows, macOS).