Mac – Time Machine backup of renamed computer fails

backuptime-capsuletime-machine

After getting a new MacBook Pro recently, I decided to reuse my old MacBook Pro as a home server. I reformatted the disk and reinstalled OS X, giving the computer a different hostname (let's call it NewHostname) in the process.

Now I want to back up the computer to the same Time Capsule I was using before. However, there is already an encrypted backup of the computer on that Time Capsule from when it was still called OldHostname. When I try to set up Time Machine, it returns an error message saying "The disk "DiskName" already contains an encrypted backup for this computer." This appears to be referring to the old sparsebundle image.

The only options Time Machine offers are to continue using the old sparsebundle or to erase it and start a new encrypted backup. Is there any way I could keep the old sparsebundle and start a new backup? And how does it even know that this encrypted file happens to correspond to this computer when the hostnames don't even match?

Note: I've already tried renaming the file in Finder from OldHostname.sparsebundle to OldHostname-old.sparsebundle, but that didn't work.

Best Answer

Try to use tmutil inheritbackup:

sudo tmutil inheritbackup /Volumes/Data/OldHostname.sparsebundle

From man tmutil:

inheritbackup {machine_directory | sparsebundle}
        Claim a machine directory or sparsebundle for use by the current
        machine. Requires root privileges.

        Machine directories and sparsebundles are owned by one computer
        at a time, and are tracked by unique identifiers rather than com-
        puter name, host name, or ethernet address. The inheritbackup
        verb reassigns the identity of the specified item, reconfiguring
        it so the current host recognizes it during backups. When inher-
        iting a sparsebundle, the machine directory within will also be
        claimed.

        Inheriting is typically only one step in the process of configur-
        ing a backup for use by a machine. You may also need to use
        setdestination, associatedisk, or both, depending on the situa-
        tion.

        One machine can own multiple machine directories and sparsebun-
        dles, but it is ill-advised for them to reside in the same place.
        In such a situation, which will be chosen during a backup is
        undefined. As a result, inheritbackup will attempt to detect pos-
        sible identity collisions before making changes.

Modes and ownership

Ideally, use chmod(1) and chown(8) to have the mode and ownership, of the copy of the disk image, consistent with how Mountain Lion normally creates a local disk image for Time Machine.

Example

GPES3E-gjp4-1:~ gjp22$ sudo ls -al /Volumes/tall/com.apple.backupd/GPES3E-gjp4-1.sparsebundle/
Password:
total 16672
drwx------@ 3 root   staff     10  7 Jun 19:07 .
drwxr-xr-x  8 gjp22  staff     10  6 Jun 18:57 ..
-rw-r--r--  1 root   staff    500  7 Jun 18:03 Info.bckup
-rw-r--r--  1 root   staff    500  7 Jun 18:03 Info.plist
drwx------  2 root   staff  78855  7 Jun 19:07 bands
-rw-r--r--  1 root   staff    444  7 Jun 18:02 com.apple.TimeMachine.MachineID.bckup
-rw-r--r--  1 root   staff    444  7 Jun 18:02 com.apple.TimeMachine.MachineID.plist
-rw-r--r--  1 root   staff   1473  6 Jun 21:28 com.apple.TimeMachine.Results.plist
-rw-r--r--  1 root   staff  10698  6 Jun 22:24 com.apple.TimeMachine.SnapshotHistory.plist
-rwx------  1 root   staff      0 19 Mar 19:36 token

– and for each band, you should have:

-rw-------  1 root  staff

Attaching the copy of the disk image

With Terminal, for the example above:

sudo hdiutil attach -readonly /Volumes/tall/com.apple.backupd/GPES3E-gjp4-1.sparsebundle/

If you use Terminal from within Recovery OS: you'll probably not need the sudo prefix to the command.

Additional detail

… whether I can still restore …

You require:

a copy of a Time Machine destination, a copy that will be good for restoration
not an additional Time Machine destination.

Assumption

You wish the copy to be usable, for restoration only, in the event of loss or failure of the Time Capsule.

If the disk is attached from a local image where modes and ownership vary from the norm for a local image:

OS X or Recovery OS might use that disk without difficulty; I can't be certain.

Thoughts

If you're preparing for the possibility of loss or failure of a Time Capsule – and restoration from a copy of what was there – then think a step further, to backing up without the original Time Capsule. (If you wish the copy to be writeable by Time Machine, that could be a separate question.)

All things considered

It's probably quicker and simpler to use the external hard disk, or a part, as a Time Machine destination.

Mac – Yosemite Time Machine Backup Failed Due to Full Disk

This somewhat of a long shot, but it sounds a little like you may have changed something on your disk. "Invisible" actions like changing file permissions or HFS+ compressing files will cause TimeMachine (TM) to assume that the files have changed and need to be backed up. Another scenario would be that the UUID of your hard disk changed. This article lists some further possibilities. So to answer your questions:

the warning is there because you would loose all previous backups and you would be left with only this one backup.
Time Machine thinks that all files have changed. My hypothesis is that you did some action changing attributes of all files or your disk's UUID.
you could try to inherit the files

I suggest you try the following:

Compare your disk's UUID with the one used in your Time Machine:
1. diskutil list and
2. look for a hidden file called com.apple.backupd.SnapshotVolumeUUID on your TM disk (not too sure about the file name, because I use sparsebundles)
Full Reset of Time Machine: Read this article for details. Basically you will have to delete /Library/Preferences/com.apple.TimeMachine.plist
Reconnect your backup. Read this article thoroughly. You will have to use either of these commands:
- sudo tmutil inheritbackup /Volumes/myTM/Backups.backupdb/myMac or
- sudo tmutil associatedisk -a / /Volumes/myTM/Backups.backupdb/myMac/myDisk

Note that you might loose your entire backup. Please make a copy of your Time Machine backup.