Time Machine disk full error—not deleting oldest backups

Need the EXACT error message, please.

Sometimes we find that message is telling you the SOURCE disk is full, and that leaves no room to build the data structures required to build a new backup instance.

How much space is free on your SOURCE disk?

Hold the Option key while you click on the Time machine icon on the Menubar and choose 'Verify Backups'.

I had EXACTLY the same issue as described by BATman4God on Oct 27, 2023. I have a similar MacBook Pro (2019) with OS Sonoma. I had the backup limit set to 1TB (which you do on the initial setup of the backup destination drive). Time Machine quit backing up with an error message saying that the destination drive was full. I thought that Time Machine would overwrite itself automatically when this situation occurred. I just got off the phone with an Apple Technician. He was concerned with my issue. When the backup space was nearing full, I should have received a message asking how I would like to manage the space, moving forward. At that time I would select file deletion options to regain space on the destination drive. I never received any such message. The Apple Technician then did a screen sharing and performed the following to hopefully solve this problem, moving forward:

In the Root Directory, he removed: 1. Cache; 2. Cookies 3. Saved Application State and 4. Time Machine Preference File. (I'm glad he was directing me, because I didn't have a clue what he was doing). I have now increased the backup limit to 1.5TB. It is backing up normally, but only time will tell if the Technician's solution will work. There is no way to prove it in advance. So far, so good...

Larry

Then Time machine has the drive reserved for backup.

You may have to literally do what it says. You might be able to turn off time machine backups and do the verify, but verify is just a 'good thing to try' NOT a known solution.

You can ADD an additional Time Machine destination drive, and backups will alternate -- every-other backup goes to every other drive. Each drive is a stand-alone backup, independent of all others and can be discontinued whiner you wish.

Drive size for long-term trouble-free operation is over 2.5 times the size of what needs to be saved. You did not say what you basic size is, but an incremental backup of the size you have now makes it sound like you are backing up a LOT of stuff.

<<. I'm confused by the statement, "Then Time machine has the drive reserved for backup." The external USB is my Time Machine backup disk, so yes, it is wholly reserved for backup. Isn't this to be expected? >>

Time machine has decided the drive is too busy right now for you to Verify it, a major step that can only be performed when the drive is not at all engaged in making a backup.

If you really need an answer, contact Support.

Official Apple Support

Whether they can find an answer or not, the solution is likely to be exactly what the error message suggested -- get a larger drive.

I agree that based on its specs, Time Machine SHOULD be consolidating backups and providing enough space to make an incremental backup, Even if yours is rather large.

If it is not able to do that in your case, you may have uncovered a Bug. It could be that it simply can't handle the case where so much needs to be consolidated in a short time to make so much room.