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I've been using Time Machine to backup to an external hard drive. Recently I got an error saying that TM could no longer back up to the backup volume, because it was read-only. (I assume it's read-only because something happened to the filesystem. Rebooting didn't fix the fact that the backup volume is read-only.) The error message said I should back up those TM backups, but it didn't have any link or hint about how to do so. (I can't find a way to recover the error message for any more details.)

So I've got another external hard drive, of the same size, to copy the TM backups to. Various guides online say to copy the Backups.backupdb folder from the old drive to the new drive. Sounds simple enough. But when I try to do so:

  • Dragging the folder in Finder, or using Copy and Paste, results in the error message "The volume is the wrong format for a backup."
    • I didn't ask for a backup at this point; I'm just copying files. But maybe macOS is trying to be extra-helpful here and help detect and prevent a future problem. Too bad it doesn't tell me what formats are acceptable or unacceptable.
  • The destination volume is APFS, which is the only format that Disk Utility currently offers for creating volumes.
  • The old backup volume is HFS. Surely it's not a problem to copy files from HFS to APFS? Is it a problem to migrate backups from HFS to APFS? If so, how am I supposed to migrate my TM backup using a recent version of macOS?
    • Is there a way to create an HFS volume on my destination drive, in Ventura 13.4? Again, the only format options offered by Disk Utility are variants of APFS.
    • Given that HFS is old and HFS+ has been standard for a long time, someone could reasonably ask whether my source drive is actually HFS+. All I can say is that diskutil reports Apple_HFS, Disk Utility says "HFS Case-sensitive Not Journaled", and mount calls it hfs. Whether these labels are inclusive of HFS+, I don't know.
  • Surely "The volume is the wrong format for a backup" isn't referring to the old TM backup volume -- TM has been backing up to it fine until a couple of days ago.
    • However ... I did update the OS to Ventura 13.4 a couple of days ago. Is it possible that this update dropped TM support for HFS, and didn't provide a warning or a migration path? I'm not sure which came first, the update or the "backup volume is read-only" problem.
  • In case anybody asks about turning off "Ignore ownership" on this volume, which is something you're supposed to do before copying the backup to a new drive: The Finder "Get info" dialog doesn't have a checkbox for this for the destination drive (and no indication of why not). I worked around this using sudo diskutil enableOwnership disk5s2, which is the destination volume. It responded File system user/group ownership enabled, so that was apparently successful, but it still won't let me copy the Backup.backupdb folder to that volume.
  • In case it helps, I'm running Disk Utility's "First Aid" process on the old backup drive. No errors reported yet, still checking catalog file.
  • There are conflicting rumors that it is impossible, or possible, to continue TM backups from an HFS volume to an APFS volume. Can anyone confirm whether this alleged FS type incompatibility is the cause of my being able to copy backup files from one drive to another?
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Time Machine on Ventura (and Monterey) uses APFS format by default. This is much more robust than continuing to use HFS+ formatted backup disks.

In answer to some of your questions:

  • You can't copy an HFS+ backup disk to an APFS disk because HFS+ backups use hard linked directories which are not part of APFS. At best you could copy a specific backup to a folder on an APFS disk.

  • Disk Utility does allow creation of HFS+ partitions.

  • You can't "continue" (whatever that means) TM backups from an HFS+ backup disk to an APFS one.

Rather than messing about with your HFS+ backup, I suggest:

  1. Keep the HFS+ backup safe in case of some future need to recover files.
  2. Use your new disk for new Time Machine backups which will be formatted APFS.
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  • OK, this is helpful. Re "Disk utility does allow creation of HFS+ partitions" - Interesting. Since my old backup is on an HFS volume, and there is no partition there identified as HFS, it didn't occur to me that any sort of partition change was needed, or that there was such thing as an HFS(+) partition.
    – LarsH
    Commented Jun 16, 2023 at 1:33
  • I would agree with your suggestion at the end, except that the old backup is on a drive with other storage, which I need to have frequent access to, whereas the other, smaller disk is a separate drive. I don't want to have to keep two external drives connected all the time. If I could move the old backup to the other drive, I'd be glad to just keep it around in case of future need, while doing TM backups to the same old drive, on a volume which I will have formatted AFPS.
    – LarsH
    Commented Jun 16, 2023 at 1:39
  • In HFS/APFS world, A drive has partitions which may be formatted a) as an HFS partition which can also be described as a single volume or b) as an APFS container/partition which can have multiple APFS volumes inside. An HFS partition/volume can have multiple folders inside (one of which might be TM backup). With APFS a TM backup occupies a whole volume (but you can have multiple volumes in the container.
    – Gilby
    Commented Jun 16, 2023 at 5:38
  • I get where you are at and where you want to end up. On the old disk, do you have one partition with data and backup in different folders? Or are they in two separate HFS partitions? And what is the size of data, of the TM backup (both total size and size of the latest successful backup) and of the two physical disks?
    – Gilby
    Commented Jun 16, 2023 at 5:46
  • I really appreciate your continued help on this, and I will respond to your questions, but please understand I don't expect you to keep walking with me through each successive development as it happens. I feel like I've already stretched the bounds of a "one question" model.
    – LarsH
    Commented Jun 16, 2023 at 13:58

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