Should I limit the Time Machine max size before it makes backups?

Question

Should I limit the Time Machine max size before it makes backups?

Hi,

One external USB disk of 4TB now holds the backups of 4 different MacBooks for me. One of the 4 is now claiming 3,5TB. The others can't backup anymore. I want to limit the size of the backup for that MacBook (only 500Gb big after all) with:

sudo defaults write /Library/Preferences/com.apple.TimeMachine MaxSize 1024000

but I am hesitant to do it right away. Should I first manually delete the backups for that machine? Then set the limit and then make new backups? (don't worry, I have another USB drive with identical backups) Or can I right away limit the size and TimeMachine will behave and properly clean up a lot of old backups? I have no experience with this, you might have!

BTW, I know that some of you recommend to have a different drive for each machine because disks are cheap. In my situation not practical. After all, I want to swap one disk with the identical duplicate I store on another location. Not swap 4 with another 4. In practise, my mother bringing one disk in her hand bag when visiting.

MacBook Air 15″, macOS 15.3

Posted on Mar 29, 2025 3:16 PM

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Answer 1

Barney-15E

Level 10

118,890 points

Mar 31, 2025 2:38 PM in response to ReindeR Rustema

I would never limit Time Machine size. Let it use whatever it needs.

If you wish to store multiple backups on the same drive, create APFS Volumes for each backup set. All will use as much space as needed up to the total drive space available.

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Answer 2

Apr 1, 2025 10:47 PM in response to Barney-15E

How to proceed?

I should not limit the size of a Time Machine backup
I should format the disk to create limited volumes for each the backups
I have to rescue or redo backups before I can create an APFS volume so I can do that
I can't move the one volume I can't redo the next months.

Can I copy that one backup with Carbon Copy Cloner perhaps?

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Answer 3

Apr 1, 2025 1:19 PM in response to Zurarczurx

Now I am stuck. I deleted three of the four backups, anticipating that I can make fresh backups after I "create APFS Volumes for each backup set" as Barney-15E suggested.

But there is one Time Machine backup I have to keep because the machine is stored elsewhere, out of reach. I try to move that backup to either my desktop or to another usb drive. I respectively get these errors:

How to rescue that one backup? It comes from a Mac OS Extended (Case-sensitive, Journaled) volume and my MacBook is APFS (what else?) and the other old disk is Mac OS Extended (Journaled). All the instructions I find online are about a single backup for one machine that needs to move to a new empty disk. Help!

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Answer 4

Apr 1, 2025 5:15 AM in response to Zurarczurx

Yes, identical disk on another location. Every month or less I swap the disks. And every summer also with the one at the holiday location.

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Answer 5

tbirdvet

Level 6

15,635 points

Mar 30, 2025 6:49 AM in response to ReindeR Rustema

You can erase and start over but TM will normally delete the oldest backup first if the drive is full so it can continue backing up. I let TM decide on how to backup (small or large backups). I have never had any issues.

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Answer 6

Mar 31, 2025 10:31 PM in response to tbirdvet

I try to limit the size of the TimeMachine backup that my MacBook makes to 1Tb. It now takes 3,6Tb on the 4Tb drive and other machines also need to make backups to the same volume. I found this command elsewhere on this forum. Perhaps I should formulate the command differently?

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Answer 7

Barney-15E

Level 10

118,890 points

Apr 1, 2025 4:18 AM in response to ReindeR Rustema

ReindeR Rustema wrote:

I try to limit the size of the TimeMachine backup that my MacBook makes to 1Tb. It now takes 3,6Tb on the 4Tb drive and other machines also need to make backups to the same volume. I found this command elsewhere on this forum. Perhaps I should formulate the command differently?

How much data are you backing up. If it’s one and a half to 2 TB of data then 3.6 makes sense. If not, then something’s wrong and I would erase the back up and start over. If you’re backing up monolithic databases, or virtual machines, then that might explain the large size and that is not recommended with Time Machine.

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Answer 8

tbirdvet

Level 6

15,635 points

Mar 29, 2025 3:44 PM in response to ReindeR Rustema

I will only say never try to delete any backup file manually from TM drive as it will mess up the system. I always let TM do its own managing of backups!

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Answer 9

Mar 29, 2025 3:47 PM in response to tbirdvet

delete that entire backup and start from scratch (but then with a max) or just the command about the max and see the system properly dealing with the surplus of backups?

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Answer 10

Mar 30, 2025 10:25 PM in response to ReindeR Rustema

The command actually gives an error:

2025-03-31 07:23:02.794 defaults[68251:5342435] Could not write domain /Library/Preferences/com.apple.TimeMachine; exiting

What to do?

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Answer 11

tbirdvet

Level 6

15,635 points

Mar 31, 2025 11:14 AM in response to ReindeR Rustema

What are you trying to do?

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Answer 12

Zurarczurx

Level 4

2,470 points

Apr 1, 2025 4:27 AM in response to ReindeR Rustema

When this disc dies - all discs die - you lose backups for 4 Macs. Have you got other backups for all those Macs?

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Answer 13

Apr 1, 2025 5:13 AM in response to Barney-15E

Storage is 318Gb. No virtual machines currently, no databases (except Mail and Photos). Will delete the backup for this one, create APFS Volumes and then backup again.

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Answer 14

Barney-15E

Level 10

118,890 points

Apr 1, 2025 2:15 PM in response to ReindeR Rustema

It is not possible to copy or move a Time Machine back up.

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Should I limit the Time Machine max size before it makes backups?