MacBook – Time machine/capsule read-only error

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I just got a new logic board installed on my Macbook Air (running OS X 10.9.4). Before the repair, I used a time capsule for backup (which has plenty of storage left). When my newly repaired MBA came home from the shop and tried to complete a time machine backup to the capsule, a message popped up that asked if I wanted to restore my previous backups. I selected that option, but it did not complete the backup and now I get an error message that says, "time machine cannot complete the backup because the disk is read only". I performed some simple troubleshooting (i.e., turned time machine off and unplugged the capsule), but no luck.

I read on the Apple support page that I should try mounting the time capsule to my desktop and ejecting it from disk utility. If "mount" means connect, I cannot do that because it usually connects wirelessly and is not showing up in disk utility. (I don't know if it is relevant, but I use the time capsule for wireless internet, and it works just fine.)

Can anyone explain to me (in the simplest, non-jargony terms possible) how I might troubleshoot this problem? I appreciate your help!

Best Answer

Read only is a fail safe mode for the backups. It means there was a problem detected with the backup and you can only restore from it.

The system won't continue adding files since that would risk your losing even the ability to recover the files.

There is a procedure to re-attach a Mac to the existing backup once logic boards are replaced, but they assume that there isn't a failure of the backup at the same time.

You should have support from the people who replaced the logic board to help make sure you aren't just missing the steps to re-establish a backup, but if the backup were mine, I'd try backing up to a blank new USB drive and get a good backup now and then troubleshoot the Time Capsule to remedy the error causing the read-only status without risking your only backup. (Since the troubleshooting is to use Disk Utility to repair or erase the Time Capsule and then start over with an initial backup).

Update: Now Officially Supported

At the time this was written (May 2013), the answer below was correct. Apple introduced new hardware, the Airport ExtremeBase Station (802.11ac) (6th generation) in June of 2013, and backing up to an external drive connected to an AirPort Extreme Base Station (802.11ac) or later AirPort Time Capsule is officially supported. Unfortunately, Apple no longer makes the Time Capsule or AirPort extreme. Also note that while for a long time Apple only supported network backups using AFP (Apple Filing Protocol), they have dropped AFP in favor of current SMB (that is, SMB3) protocols. This support article seems to be getting updates as the support situation evolves.

So, in 2019, your options for wireless backups appear to be a NAS with SMB3 support and advertised Time Machine compatibility or a Mac shared as a Time Machine backup destination (see support article linked above).

Original Answer

It is not officially supported. "Time Machine can’t backup to an external drive that's connected to an AirPort Extreme, Time Capsule, or a drive formatted for Microsoft Windows (NTFS or FAT format). (Click on "Setting up Time Machine backups using an external drive" to reveal the information.)(Update August 2019: the original link now redirects you to current documentation. For historical information, see also Disks that can be used with Time Machine) This is usually because of issues with the network file system not guaranteeing that writes have completed on disk before giving a "success" status.

Mac – Make backup of Time Machine volume on Time Capsule

Can I copy those .sparsebundle files to an external hard disk and expect them to work as Time Machine backup resources again?

Almost certainly yes.

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.