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.
Best Answer
My experience is netatalk is fine until the volume is full and Time Machine starts removing old backups selectively. The deletion process (which involves unlinking a whole load of hard links) seems to be the point at which netatalk falls over and you get the dreaded 'verification' message.
From what I can tell, people who say "it works fine" generally haven't got to the point where their disk is full.
However even external hard drives connected to a AirportExtreme are not immune to this error message. The only completely reliable backup systems I have used are the internal hard drive in an AirportExtreme, and a USB hard drive connected locally.