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I'm using Catalina on an formatted APFS SSD and am finding a correct way to backup the OS. In the plugged in external HDD, I create a backup volume with APFS format using Disk Utility. But when I choose it as the backup volume in Time Machine, Time Machine requires the volume have to be erased. I did it and then go to Disk Utility to check, the volume is now formatted as Mac OS Extended (Journaled).

Should I continue to use it? Will it occur any problems when I make a restore from backups on a Mac OS Extended (Journaled) to an APFS SSD? Thanks for any help.

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While, in general, APFS is better suited for SSD disks and HFS+ (macOS Extended) is better suited for rotational media due to their respective statistical access patterns and ensuing consequences for seek time and flash wear, either format could in principle be used for backup purposes if Time Machine has enabled support for it in a given macOS release. In theory*, restoring from an HFS+ to an APFS volume (or vice versa) should present no problem, as the file copy is done at a higher abstraction layer in the call stack that is agnostic to the underlying storage format. Neither format has inherent limitations that would prevent it from being used as a TM backup and it's merely a question of application (i.e., Time Machine) support. Refer to Marc Wilson's insightful answer for details on which macOS version has enabled support for which formats.

* — The difference between theory and practice is that, in theory, there is no difference between theory and practice, and in practice, there is.

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    The difference between the theory & practise in this case is that no matter what the theory, in practise Catalina will only use HFS+ disks for Time Machine;)
    – Tetsujin
    Mar 19, 2021 at 7:57
  • I am sorry, but this is at least partly wrong. The answer from Marc Wilson is correct.
    – Gilby
    Mar 19, 2021 at 9:50
  • @Gilby I've reworded my answer to refer to Marc Wilson's but I don't believe that any portion of mine was incorrect before or after my edit. Please show me any inaccuracy that you find.
    – pion
    Mar 20, 2021 at 0:53
  • @pion Your first sentence is I believe wrong for most use cases and certainly for backup. In the specific case of TM, Big Sur backups to HDD are best to use APFS based on my experience. Your digression re 'in theory' doesn't help. Better just to say "Restoring from an HFS+ to an APFS volume (or vice versa) presents no problem, as..." for the reason you have given.
    – Gilby
    Mar 20, 2021 at 2:24
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    The 'theory' falls down drastically when it comes to Time Machine, because until Big Sur it relied very heavily on hard links, which APFS cannot handle. This makes the answer at the least misleading. Time Machine before Big Sur absolutely cannot use APFS, whatever other theory may support it.
    – Tetsujin
    Mar 20, 2021 at 9:45
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Time Machine in Catalina only supports HFS+ as a destination volume. The first macOS that supports APFS as a destination volume is Big Sur.

ref:

https://support.apple.com/guide/mac-help/types-of-disks-you-can-use-with-time-machine-mh15139/10.15/mac/10.15

vs:

https://support.apple.com/guide/mac-help/types-of-disks-you-can-use-with-time-machine-mh15139/11.0/mac/11.0

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  • And in my experience, Big Sur Time Machine backing up to APFS is much superior (speed, reliability, usability) to backing up to HFS+. This is true whether the destination is HDD or SSD.
    – Gilby
    Mar 19, 2021 at 9:53
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    That, of course, requires you to run Big Sur, and who wants to do that at this point if you don't have to?
    – Marc Wilson
    Mar 19, 2021 at 16:17

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