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User profile for user: russell159
russell159 Author
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problems setting up disk to use with time machine

I have a set up with 3 Macs. I have a sata docking station attached to one of them which I use for time machine. This allows me to easily take rotate physical disk for off site storage. I have used this system for years. I back up multiple machines to one disk.


I recently add a new disk and followed instructions (that I now can't find) to format the disk as APFS (encrypted) shared it with the other machine on the network and did a backup. So far so good.


I now tried to back up the machine that had the disk directly attached. When I tried to select the disk in Timemachine prefs I got the familiar prompt "Use new disk or both", selected both and was then told that I had to erase the disk!



one can see the here:

1/ the formatting information from disk utility

2/ the dialogue from Time Machine prefs


In fact I have been around this process several times : (. I have tried backing up the local machine first and then it won't let me backup the remote one. I have tried formatting the disk with Journaled... like my older disk, but nothing works for both machines.


side note: As mentioned this all used to work fine. About a year/ 18 months ago it the disk sharing broke after a major OS upgrade (I can't remember which). I eventually concluded that the issue was that the UIDs (unix numeric ids) of account were different. Fixing this is not straight forward and I put it off and eventually did it recently. I suspect the change was a consequence of Apple attempt to make it more difficult for malware to modify files.


Suggestions solicited!

iPad mini 4, iPadOS 14

Posted on Jul 6, 2021 1:39 PM

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User profile for user: Tesserax
Tesserax
User level: Level 10
148,871 points

Posted on Jul 7, 2021 7:31 AM

russell159 wrote:

Are you saying that one can't have one disk on a mac and use it to back up the local machine and other remote ones?

No, not at all. I'm saying the following with a shared backup drive directly connected to Mac 1:

  • TM on Mac 1 will store TM backups in a different format, than Macs 2-x that connect to Mac 1's shared drive.
  • TM uses backup folders for backups to locally-attached drives.
    • Ex: /Volumes/DriveShareName/Backups.backupdb/ComputerName/
  • TM uses sparsebundles for backups to network drives.
    • Ex: /Volumes/Time Machine Backups/Backups.backupdb/ComputerName.sparsebundle


So, with that said, let's use an example how this can be problematic. We will use your configuration for this example. Let's say we have two Macs: Mac 1 & Mac 2, with a shared external drive attached to Mac 1. In theory, both Macs should be able to use TM, respectively, to make backups to that drive ... but again in different formats. The problem will arise if you decide to remove this external drive, and then, attach it directly to Mac 2. The next time TM runs it will not see its previous sparsebundle backup set and ask to start a new one. At least, this is been my experience over the years.


The bottom line is that TM was designed to use a "fixed" backup destination. Any other configuration could potentially lead to backup corruption requiring starting a new backup set.

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User profile for user: Tesserax
Tesserax
User level: Level 10
148,871 points

Jul 7, 2021 7:31 AM in response to russell159

russell159 wrote:

Are you saying that one can't have one disk on a mac and use it to back up the local machine and other remote ones?

No, not at all. I'm saying the following with a shared backup drive directly connected to Mac 1:

  • TM on Mac 1 will store TM backups in a different format, than Macs 2-x that connect to Mac 1's shared drive.
  • TM uses backup folders for backups to locally-attached drives.
    • Ex: /Volumes/DriveShareName/Backups.backupdb/ComputerName/
  • TM uses sparsebundles for backups to network drives.
    • Ex: /Volumes/Time Machine Backups/Backups.backupdb/ComputerName.sparsebundle


So, with that said, let's use an example how this can be problematic. We will use your configuration for this example. Let's say we have two Macs: Mac 1 & Mac 2, with a shared external drive attached to Mac 1. In theory, both Macs should be able to use TM, respectively, to make backups to that drive ... but again in different formats. The problem will arise if you decide to remove this external drive, and then, attach it directly to Mac 2. The next time TM runs it will not see its previous sparsebundle backup set and ask to start a new one. At least, this is been my experience over the years.


The bottom line is that TM was designed to use a "fixed" backup destination. Any other configuration could potentially lead to backup corruption requiring starting a new backup set.

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User profile for user: Tesserax
Tesserax
User level: Level 10
148,871 points

Jul 6, 2021 2:23 PM in response to russell159

I think this comes down to that Time Machine (TM) uses a different process between locally-attached drives and networked drives. A shared drive would be considered the latter. So for example, if you first make a TM backup to a locally attached drive, then attempt to update it with the next incremental backup, with the drive being shared from another Mac, TM will see this as a network drive and request to start a new backup. AFAIK, this has been true since TM was first introduced with OS X Leopard.

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User profile for user: russell159
russell159 Author
User level: Level 1
11 points

Jul 6, 2021 5:31 PM in response to Tesserax

Thanks for your response Tesserax


This worked fine until I installed Catilina ( I just check back to see when the problems started -- I thought it was longer) .


Are you saying that one can't have one disk on a mac and use it to back up the local machine and other remote ones? There are quite a few articles out there from reputable sources about backing up multiple machines to a shared volume and none of them mention that you can't back up the local machine to that disk.


That certainly appears to be the case now. I have another machine which I back up to a different disk that I had mounted locally. Once I fixed the UID issue that stopped the sharing I moved it back to the original host but backups to it are now failing with the error "he network backup disk does not support the required capabilities."


I am now looking at using volumes on the disk -- one for the local machine and one for remote ones.



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User profile for user: steve626
steve626
User level: Level 6
10,175 points

Jul 7, 2021 1:31 PM in response to russell159

Tesserax is correct, I believe, regarding the way you are trying to share resources and perform backups, with multiple disks/machines configured as you have.


In addition, Catalina does not back up properly to an APFS disk, it requires HFS+ for Time Machine. Normally it won't back up at all to APFS disks but it certainly won't work properly if it does.


Starting with Big Sur, one can back up to APFS disks with Time Machine.

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User profile for user: steve626
steve626
User level: Level 6
10,175 points

Jul 9, 2021 9:11 PM in response to russell159

russell159 wrote:

Thanks for that! That answers my next question. Now that I have a second disk to back up the host where the disks are mounted why is time machine insisting on reformatting it from APFS! ? :)

OK I will upgrade this machine to Big Sur and try again!

Catalina Time Machine backups have to be backed up to HFS+ disks. With Big Sur you can back up to APFS disks. So if you did complete the Big Sur upgrade, the APFS disk should now work.

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problems setting up disk to use with time machine

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