"The identity of the backup disk has changed..."

Question

About 3 months ago I purchased my new MacBook Pro plus two WD 2TB network drives for Time Machine backups. One drive is kept at home and the other is at my office. (I believe in having backups at two physical locations.)
The drives at both places are identical, with identical names and in both cases are connected to a BT router/DSL modem.
All has been working nearly perfectly. (I say almost, because occasionally at home I'll get an error saying a backup couldn't be made because the disk wasn't available. Usually this clears up after a few subsequent attempts, or always after a reboot. Fortunately this doesn't happen often.)
However, I'm now getting an error message every time I move from one location to the other:
"The identity of the backup disk has changed since the previous backup.
The disk may have been replaced or erased, or someone may be trying to trick your computer into backing up to the wrong disk."
Fortunately, the dialog box is very visible and gives the option of going ahead and using the disk...which I do. As long as I'm in the same location, everything then continues to work without issue.
This is most odd, as neither disk name has been changed and I was working for 3 months without anything like this happening.
(I might add, with my previous Powerbook, I had an identical Firewire drive at both locations and having the dual backups worked flawlessly without issue for the entire time I used it...nearly two years.)
Anyone have any idea why this weird message is now surfacing after 3 months of working fine?

Answer 1

Pondini wrote:
There was a thread here a few months ago with a very nasty problem, where a user had 3 or 4 identical WD externals, with different names, and was trying to use one with Time Machine. But Time Machine would try to back up to the first one connected, regardless of name!
I didn't think it was possible, but sure enough, they all had the identical UUID from WD (they'd come formatted for a Mac, and the user had never reformatted or erased them, so OSX never put new UUIDs on them).
I wonder if the UUID would be copied over if a disk is cloned? When using my first pair of external LaCie Firewire drives, I first set just one up as a TM backup. After the setup and first backup was finished, I used Carbon Copy Cloner to create an exact duplicate to the second drive. I did that deliberately in hopes TM wouldn't be able to tell the difference. It apparently worked as I could connect either disk and Time Machine would readily connect without complaint. Either the cloning procedure worked, or perhaps the drives already had identical UUID's as happened with the WD drives you mentioned. Any thoughts on this?
Pondini wrote:
If you haven't, try turning Time Machine OFF before you get that message. Then select the NAS via Time Machine Preferences > Select Disk. I think that will reset the element in the plist mentioned in that thread, and the problem will go away.
I went ahead and tried this, but the condition still persists. When I have a chance, I'll go back and look at that other thread and see if I can glean anything useful from it. In the meantime, I may just have to live with the current situation. Thanks.