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

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

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?

2.66 GHz 15" MacBook Pro i7, Mac OS X (10.6.5), 8 GB RAM

Posted on Nov 20, 2010 5:00 AM

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Answer 1

Pondini

Level 8

38,863 points

Nov 20, 2010 6:46 AM in response to Douglas Tews

You're doing the "swap" via an automated process, right?

If so, see: http://discussions.apple.com/thread.jspa?threadID=2647291

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Answer 2

Douglas Tews Author

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82 points

Nov 20, 2010 7:56 AM in response to Pondini

Automated process? I'm not certain what you're referring to.

All I can say is for the last 3 months I've been able to take the laptop back and forth from work to home and Time Machine has always connected to both disks and worked with no error message. Now, I'm suddenly getting the message mentioned above. Coming to think of it, this may have coincided with the last system update, so perhaps Apple has changed something in the way Time Machine works.

The thread referred to is a bit lengthy, but I'll study it more detail, as at the end there is a suggestion there may be an answer to this. Thanks.

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Answer 3

Pondini

Level 8

38,863 points

Nov 20, 2010 8:04 AM in response to Douglas Tews

Douglas Tews wrote:
Automated process? I'm not certain what you're referring to.

How do you tell Time Machine to back up to a different location?

If you use +Time Machine Preferences > Select Disk,+ you shouldn't get that error.

If you use an Applescript or other such method, you will, until you change it, as specified towards the end of the other thread.

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Answer 4

Douglas Tews Author

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82 points

Nov 20, 2010 1:59 PM in response to Pondini

By using two identical disks with identical names, I've never had to tell Time Machine to back up to a "different location". Regardless which disk is connected, when opening the Select Disk in TM preferences, the exact same name is displayed.

Until recently, it has apparently only looked for a disk with a certain name and upon finding it, uses it. Both disks are identical models with identical names (but at different locations) and up until now Time Machine has happily backed up to which ever one is connected.

The only thing I can think of is Apple has recently added another factor in determining a disk's identity other than just its name. Perhaps it's detecting that each disk is on a different network and that may be throwing up a flag...I don't know. If so, that's a valid security measure, but it unfortunately throws a wrench in my heretofore simple system.

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Answer 5

Pondini

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38,863 points

Nov 20, 2010 2:08 PM in response to Douglas Tews

Douglas Tews wrote:
By using two identical disks with identical names, I've never had to tell Time Machine to back up to a "different location". Regardless which disk is connected, when opening the Select Disk in TM preferences, the exact same name is displayed.

Until recently, it has apparently only looked for a disk with a certain name

It's not the name, it's the UUID (Universally Unique Identifier) that OSX and Time Machine use to keep track of volumes. Each internal or external disk drive or partition has one; on a "normal" network drive, such as a Time Capsule, Time Machine will make a +sparse bundle+ for the backups.

Since you're backing-up to a NAS, the UUID may be supplied by the NAS.

Apple did make a change effective with 10.6.5, as detailed in the thread.

You may want to see if WD has any updates, and/or check their support.

In the meantime, most likely you can just re-select the destination you want.

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Answer 6

Douglas Tews Author

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82 points

Nov 20, 2010 3:21 PM in response to Pondini

Sorry, I can't remember how to properly get quotes into these forum replies...

"It's not the name, it's the UUID (Universally Unique Identifier) that OSX and Time Machine use to keep track of volumes."

I was suspicious of this and seemed to remember such a unique identifier existed. However, if this is indeed what Time Machine is using, why has my identically named disks worked perfectly up until now? As previously mentioned, I also previously used two external, identically named Firewire drives in the same fashion for almost two years and never had this issue crop up either. If Time Machine was using a unique identifier other than the name, my scenario should have never worked period, with either setup. It all seems a bit mysterious.

Fortunately, it's not a serious issue, just a bit of a momentary pest. After changing locations and the dialog appears, I simply hit "use this disk" and Time Machine does it job with no further problem at that location. Nonetheless, I'll take a closer look at that other thread and see if I can glean anything that might help me there. Thanks!

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Answer 7

Pondini

Level 8

38,863 points

Nov 20, 2010 3:30 PM in response to Douglas Tews

Douglas Tews wrote:
Sorry, I can't remember how to properly get quotes into these forum replies...

Click the double-quote symbol just above this box (when in compose mode).

"It's not the name, it's the UUID (Universally Unique Identifier) that OSX and Time Machine use to keep track of volumes."

I was suspicious of this and seemed to remember such a unique identifier existed. However, if this is indeed what Time Machine is using, why has my identically named disks worked perfectly up until now?

Ordinarily it doesn't. 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).

Fortunately, it's not a serious issue, just a bit of a momentary pest. After changing locations and the dialog appears, I simply hit "use this disk" and Time Machine does it job with no further problem at that location.

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.

If not, contact WD.

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Answer 8

Douglas Tews Author

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82 points

Nov 22, 2010 6:41 AM in response to Pondini

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.

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Answer 9

Pondini

Level 8

38,863 points

Nov 22, 2010 7:24 AM in response to Douglas Tews

Douglas Tews wrote:
. . .
I wonder if the UUID would be copied over if a disk is cloned?

No (that's kind of the point of a UUID -- you can have multiple volumes on a single Mac with the same name, but OSX knows which is which). If you erase or reformat a drive, it gets a new one. Otherwise, it doesn't. So those must have come with the same UUID. 😟

But you've got NAS devices; they have their own operating systems. I have no idea what they do.

The message you're seeing has to do with an element in the plist on the Mac that controls Time Machine; WD's programing apparently hasn't been updated to work with 10.6.5.

You might try a "full reset" of Time Machine, per #A4 in [Time Machine - Troubleshooting|http://web.me.com/pondini/Time_Machine/Troubleshooting.html] (or use the link in *User Tips* at the top of this forum).

Then set up Time Machine again using WD's instructions. That may or may not get the right things in the plist.

If still no help, contact WD.

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Answer 10

Feb 8, 2011 5:12 PM in response to Douglas Tews

I'm suddenly getting the same error on two of my three machines (all of which back up to a central WD External USB HDD hosted on an AEBS) - which have been happily backing up for a couple of months in this configuration.

The difference is: I only have one hard drive and nothing in my configuration should have changed.

I started checking the Console logs for evidence of break-ins or configuration changes to the system, but I didn't find anything out-of-the-ordinary. Any thoughts???

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Answer 11

Pondini

Level 8

38,863 points

Feb 8, 2011 6:45 PM in response to WhatsWrongWithMyMacs

WhatsWrongWithMyMacs wrote:
I'm suddenly getting the same error on two of my three machines (all of which back up to a central WD External USB HDD hosted on an AEBS) - which have been happily backing up for a couple of months in this configuration.

You need to know that backing-up that way is "iffy" and +*not supported by Apple.+* See [Using Time Machine with an Airport Extreme Air Disk|http://web.me.com/pondini/Time_Machine/Airport.html] (or use the link in *User Tips* at the top of this forum).

As noted there, and in many, many threads here and in the Airport Extreme forum, it will seem to work fine for a while (days, weeks, or even months), then suddenly the backups are corrupted, often beyond repair.

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Answer 12

May 22, 2011 3:18 PM in response to Douglas Tews

I've had a similar problem with a Iomega 1TB HD connected to a 500GB Time Capsule.

My new MacBook Pro i7 (3 months old and running 10.6.7) has been backing up across the home wifi via the Time Capsule network.

Recently the backup message has appeared that the identity has changed. It had been working OK previously and 2 other Macs in the house have been backing up to the Time Capsule as normal (not to the external HD)

When the identity is lost Time machine creates another sparse bundle and backs up the whole MBP HD again.

If you have a very large external HD then you will not notice apart from the hours it takes to make 'another' initial back up.

I visited the Genius bar with little help, apart from run repair in disk utility on the external HD.

The HD did need repairing and I performed the Disk utility repair.

After the repair and new sparse bundle back up the lost identity has happened 2 weekends in a row, again today.

This is getting frustrating and I hope Apple can fix it.

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