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I have multiple Macs backing up to a NAS. The NAS is backing up to an off site server. Problem is, the network Time Machine backups are unreliable (in my experience), often getting corrupted, and not finishing on laptops connected via WIFI. To work around these issues, I want to use a large, external USB drive that I connect to most important machines once per month or so to make a local backup. I want to share this USB drive across multiple machines vs buying a new USB drive for each machine.

In the past, I was able to just attached a USB drive and backup the Mac, then attach to the second Mac and backup to the same drive. Time Machine created a separate folder for each Mac I backed up.

For some reason, however, my new drive is not doing this. If I backup a Mac, then the next machine sees that as another machines backup and I have a choice to wipe (Start a New Back) or inherit that backup.

I tried reformatting the drive as AFS and HFS. It doesn't matter as the result is always the same.

Is there anyway to setup an external USB drive as a portal Time Machine backup drive for multiple Macs?

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  • Save each backup inside a sparse bundle.
    – benwiggy
    Commented Aug 7, 2022 at 16:45

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Time Machine (pre macOS 11) used HFS+ formatted backup disks with a different folder for each backup. But with macOS 11 and 12, new backups use APFS formatted volumes - note a volume for each backup, not a folder. From what you say, I think you have got tangled up in that transition.

For the future, you can create multiple APFS volumes on your USB drive and tell Time Machine on each Mac to use its specific volume.

As an example of disk layout, I have created a disk image, first with one APFS volume and then added a second and third volume. You can do the same thing with your USB drive, though it will require erasing (or re-partitioning) the drive to get started. Ready for 3 backups On each Mac, tell Time Machine to use its own volume. TM will reformat that volume for its use.

And you can add further volumes as required for each Mac. All the backup volumes will grow as required.

Regarding network backups, Time Machine has got more reliable with recent macOS versions though much depends on configuration of the NAS. You may want to re-explore that option.

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  • Excellent explanation and solution to my problem. I much prefer APFS anyways. I assume I can use the backups made in this way to restore a Mac from a full rebuild if necessary? Commented Aug 8, 2022 at 1:15
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    @SwisherSweet When doing a full recovery, Migration Assistant will offer you a recovery from any one of the backups.
    – Gilby
    Commented Aug 8, 2022 at 2:42
  • Thanks again Gilby. Solution is working perfectly. Do you happen to know where I might find some tips on how to configure our Synology server to make our TM backups more reliable? Commented Aug 8, 2022 at 22:01
  • Sorry, I have no experience of synology. I do know that it 'supports' TM, but that you do need to configure shares to use whatever it does. Try Synology web site?
    – Gilby
    Commented Aug 8, 2022 at 22:54

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