User profile for user: John Rose6
John Rose6 Author
User level: Level 2
351 points

Time Machine doesn't make room on backup drive, even though I have less data to back up

For Sonoma 14.6.1, there's this info from If the Time Machine backup disk for your Mac is full - Apple Support (CA) :

If the Time Machine backup disk for your Mac is full
Your first Time Machine backup includes everything on your Mac. After that, Time Machine finds and saves only new and changed items, so the backups become smaller. Also, as your backup disk fills up, Time Machine deletes older backups to make room for new ones. You may be able to use Time Machine for a long time before running out of space.
If you do run out of space, it’s best to connect a new backup disk. After you connect the new disk, open Time Machine settings, then select it as your Time Machine backup disk.

Time Machine itself says :

Time Machine backs up your computer and keeps local snapshots for the past 24 hours, daily backups for the past month and weekly backups for all previous months. The oldest backups and any local snapshots are deleted as space is needed.

Muninn HD is my MacBook Pro's drive, Clio is an external SSD formatted as APFS by Time Machine. Both are nominally 500 GB. Clio has some backups on it already.


I recently found a lot of unneeded ISO and disk image files that I copied to a USB stick and deleted from Muninn HD. This gave me lots of room for music files from an old Mac mini that I intend to copy to my iPhone.


As of this screen shot, you see that Munin has 365.5 GB of data, and Clio has 473 GB.

Since I deleted lots of data (80 GB or so) from Muninn I should have lots of room on Clio, right?


Then I get this Backup Not Completed alert:

If I now have even less data to back up, and Time Machine supposedly deletes older backups to "make room for new ones", then why won't it do some housekeeping and make room on Clio?


The last time this happened and I really wanted a current backup, I had to erase everything on Clio and start a new backup from scratch. It took forever.


Is there a way to get Time Machine to do its job as described by Apple?


P.S. — Bonus points if you know why I named them "Muninn" and "Clio" without googling them.

MacBook Pro 13″, macOS 14.6

Posted on Aug 31, 2024 6:36 PM

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6 replies
User profile for user: Bob Timmons
Bob Timmons
Community+ 2025
User level: Level 10
184,652 points

Sep 2, 2024 6:26 AM in response to John Rose6

Time Machine supposedly deletes older backups to "make room for new ones"


That's the theory, but it assumes that you have been making small "incremental" backups and are continuing to do so. If the disk is fairly full, and you make substantial changes......like updating the operating system or adding or deleting a large amount of files, Time Machine is not going to be able to delete enough data to make room for the new backup.


Since I deleted lots of data (80 GB or so) from Muninn I should have lots of room on Clio, right?


No, Clio keeps the old historical files in the backup. Just because you delete a fair amount of data on your Mac, does not mean that the data will be immediately deleted from the backup files. Remember, Time Machine keeps a history of backups.



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User profile for user: Barney-15E
Barney-15E
User level: Level 10
122,120 points

Sep 1, 2024 7:17 AM in response to John Rose6

For that to work, you need 2x to 3x the amount of space you are backing up. You should have a 1.5TB drive for your backup, but could get away with about 750GB (not that anyone makes such a thing). Get at least a terabyte.


1) APFS needs lots of free space on the drive to move things around.

2) Time Machine won't delete anything from the backup which contains things still on the drive. All versions will remain in the backup if the file still exists on the source drive. So, there may not be "older backups" that can be deleted.

As I understand the APFS snapshot backup method Time Machine uses, I don't know how it could possibly delete older backups, but I may not fully understand how a snapshot can be modified (i.e., I don't think it can). I would imagine it would have to remove the "older backup" files from the base storage, then repair all of the Snapshots. Seems this would probably require quite a bit of temporary storage.


If you erased your backup drive and started over, it wouldn't be long before you got to the point that it is filled enough that the drive cannot operate efficiently. SSDs require much more empty space than a spinning HDD to operate efficiently, so consider that in your purchasing decision. Also, you don't really need the speed of an SSD for a backup.

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User profile for user: John Rose6
John Rose6 Author
User level: Level 2
351 points

Sep 1, 2024 9:14 PM in response to Barney-15E

Barney-15E wrote:
...
2) Time Machine won't delete anything from the backup which contains things still on the drive. All versions will remain in the backup if the file still exists on the source drive. So, there may not be "older backups" that can be deleted.

The thing is that I deleted 80 GB from Muninn HD. I don't want to keep any "older backups". It should at least be able to have one complete backup which it can do for even more data, If I erase it first.

Apple is not being entirely truthful when it says it will "make room for new ones".

If you erased your backup drive and started over, it wouldn't be long before you got to the point that it is filled enough that the drive cannot operate efficiently. SSDs require much more empty space than a spinning HDD to operate efficiently, so consider that in your purchasing decision. Also, you don't really need the speed of an SSD for a backup.

It was a frankendrive.

I had a portable HDD that died, and just happened to have a spare naked SSD laying around for some reason or other. I put the SSD in the HDD's enclosure, taped it shut, reformatted it, and it worked (except for the little activity LED not working any more), so there was not really any additional expense on my part.

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Time Machine doesn't make room on backup drive, even though I have less data to back up

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