One of the external backup disks I use to back up my personal laptop filled up when I was too busy to buy another, so (in a rush) I grabbed another project's backup disk and added my laptop to it too. Months later, now that project's backup disk is full, and I have finally bought a new backup disk for my own laptop. I need to free up the space again on the project's backup disk, for the project.

I tried formatting the new disk and was surprised to find that Apple now recommends using APFS to format time machine back ups. I did this, I used the new format in reformatting the disk. Now though when I try to drag my backup from the old backup disk, I get a pop up that says "The volume is the wrong format for a backup."

Is it not possible to move old laptops to an old disk? Do I just need to backup from scratch and delete these months of backups (I do have 2 other backup disks in other locations so that's not the biggest of deals, but it still rubs me the wrong way to lose data.) Or should this work? Would it work if I just reformat the new disk with the old (deprecated?) Apple "extended journaled" format?

  • Time Machine is designed to 'always be full'. That kind of throws the rest of the question out of whack somewhat.
    – Tetsujin
    Jan 12 at 16:21
  • @Tetsujin Sorry your comment makes no sense to me, and I use time machine all the time, and have for years. It backs up to hard disks, at one point the disks become full due primarily to revisions of software by apple and/or buying a new laptop and it makes an entire new copy of the hard disk. But for some time (years?) you use the same disk and incremental backups. Jan 12 at 16:42
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    Time Machine is designed specifically to 'never run out of space'. It backs up hourly, daily, weekly until the drive is full, then recycles the oldest data to always leave room for the newest. Unless you change machine, this can theoretically run ad infinitum.
    – Tetsujin
    Jan 12 at 16:45
  • Ah, yes, I have seen it behave like that before. I must have managed to have turned that off at some point, I hate that behaviour. I would rather keep samples of older disks, it's the backups that are right near to each other that are less valuable, not the old ones. Jan 12 at 17:21
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    @Tetsujin you were right! The problem was not enough disk on the laptop for timemachine, not on the disk! But I'm leaving this up because someone may want to do this kind of migration. Jan 30 at 12:58

1 Answer 1


OK, I hacked around, it turns out that you do indeed just have to make EXACTLY the same format as the disk you are copying from for Apple to let you drag & drop between disks. I'm sure there are other ways to do it, but for now I'll just stick with Apple's disk format.

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