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I have a hard drive which has one HFS partition of 1TB that is being used as a MacBook backup.

Can I backup another MacBook to the same partition without overwriting or convoluting data?

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2 Answers 2

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Yes, the Time Machine system handles this automatically.

There's one major drawback in that you cannot decide on a storage quota for each backup. This means that if you have 1 TB of backup storage, you cannot decide that 500 GB is reserved for computer A and 500 GB for computer B.

So if the backup of one computer is too large, that could make it impossible to take a full backup of the other computer.

You can limit the size of a Time Machine backup though by executing on each Mac:

sudo defaults write /Library/Preferences/com.apple.TimeMachine MaxSize -integer size

with size in MB. Example:

sudo defaults write /Library/Preferences/com.apple.TimeMachine MaxSize -integer 500000
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  • added a TM size limiting command
    – klanomath
    Commented Jan 8, 2017 at 21:23
  • Would the folder structure reflect that there are two separate computers?
    – Kellen Stuart
    Commented Jan 9, 2017 at 7:06
  • Time Machine creates a seperate sparsebundle for each computer. They are called computer name.sparsebundle - for example "Kolobs MacBook Air.sparsebundle".
    – jksoegaard
    Commented Jan 9, 2017 at 9:42
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I would highly recommend creating a separate partition for each computer that is being backed up via Time Machine, and if required (optionally) create another separate partition for general storage.

As an example... for a 4TB drive:

Partition 1 - Time Machine Backups (Computer 1) - 500GB

Partition 2 - Time Machine Backups (Computer 2) - 500GB

Partition 3 - General Storage - 2TB (optional)

The potential issue I see with jksoegaard's recommendation is that if you ever perform a major MacOS version update, or system restore, on either or both computers - and then forget to immediately reapply the sudo command to set the MaxSize - the MacOS update may inadvertently set MaxSize back to using the full disk space (system default) and will stop applying the storage quotas... before you have a chance to realise.

Using partitions, however, you can more effectively apply a storage quota for each computer backup irrespective of MacOS updates in the future.

I found this link very helpful: http://www.baligu.com/pondini/TM/4.html

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