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I had two SSD's - when I tried to use them for time machine backups, they were automatically formatted as APFS without giving me an option to select Mac OS Extended Journaled.

The problem is, there is some kind of bug in Big Sur, where randomly after a few backups there is an error during the "Preparing Backup" stage, where it just says there was an error (no detailed information). This has been heavily documented on Apple forums.

I have found that there is nothing wrong with the drives, and they work fine when formatted as Mac OS Extended Journaled. But Big Sur does not give me the option to create my time machine drive with HFS+. It automatically chooses APFS, and reformats my drive to APFS if I format it to HFS+ first!!

At first I thought this was just for SSD's, but now I find it is doing the same when I try to backup to an ordinary HDD.

How can I prevent this from happening? This seems to be some bug in Big Sur Time Machine backups. I cannot update to Monterrey because I have an old Mac which does not support further updates.

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    I'm not convinced this problem is limited to APFS formatted drives. I have run into it several times with time machine backups to a NAS (e.g. over SMB, the NAS itself is formatted with brtfs).
    – negacao
    Jan 1, 2022 at 15:32

2 Answers 2

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In addition to Kaizer Sozay's answer:

you do not have to make a backup on another machine. It's enough to put the following folders on the drive after formatting it with HFS+ (Mac OS Extended Journaled):

Backups.backupdb |__ SomeMachineName |__ 2022-07-30-060606

It's best to create this three nested directories/folders on your desktop first and copy it to your HFS+ TimeMachine drive in one go, because if you create a folder called Backups.backupdb on the root level of a drive, your Mac won't let you copy anything to or create in that folder.

These three folders are all it takes to convince Monterey that your drive is a valid TimeMachine volume and it will not reformat it with APFS when you add it to TimeMachine BigSur or Monterey.

IMHO it only makes sense to go through this effort for a spinning harddisk. If you use an SSD for TimeMachine, APFS is indeed the better choice.

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    /Volumes/HFSDrive/Backups.backupdb/SomeMachineName/2022-07-30-060606 worked for me! Thanks! For those wondering, this seems highly worth it given how poorly APFS works on HDDs.
    – JBis
    Aug 1 at 20:43
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  1. First find a machine running an earlier version of Mac OS prior to Big Sur.
  2. Format your SSD for HFS+ (Mac OS Extended etc)....
  3. Create time machine backup of this earlier OS.
  4. Then take the same drive to your Big Sur or later machine, and use the same drive to backup. This time, Time Machine will not wipe / format your drive as APFS because it will detect that there already is a Time Machine backup stored on the drive. As an added bonus, you can use the drive to store other files on it as well!

EDIT: Here is where its mentioned: https://support.apple.com/sq-al/guide/mac-help/mh15139/11.0/mac/11.0

The key thing is, the drive should already contain a time machine backup. If it does, then Time Machine won't wipe it / format to APFS, and will just continue with the previous format (Mac OS Extended)

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  • It looks like this is a workaround by Apple to provide backwards compatibility with existing HFS+ Time Machines. As per this link, APFS conversion is done by de-facto for brand new TMs and most importantly, it cannot be disabled either…
    – Alexander K.
    Mar 7, 2022 at 17:57
  • 👆 Very cool hint. Apple's exact wording: If the disk is a Mac OS Extended format disk that contains an existing Time Machine backup, you aren’t asked to erase and reformat the disk. I had thought from research and all my experiments that macOS 11 Big Sur simply has read-only support for TimeMachine backups on HFS+ volumes. But with an inherited HFS+ disk (itself not enough) + a TimeMachine backup on it backing up that very same current Mac, you can continue TM with r/w access! Setting this up in retrospect on a newer macOS is impossible: → See next comment.
    – porg
    Apr 27 at 10:04
  • So in practise this will apply only to a handful of people. Setting this up in retrospect on a newer macOS? De facto impossible for this reasons: Using a boot disk won't work. Can only create a fresh HFS+ volume (with FileVault 2 encryption if you want too). But can not set up TimeMachine on it. That can only happen if you are booted into the disk which you are backing up (your main storage device). Except maybe with extreme hacking (diskutil + hacking volume-UID files manually, etc) → 0% practical chance. macOS Big Sur: Using TimeMachine requires APFS for the majority.
    – porg
    Apr 27 at 10:08

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