How to find iPad or iPhone device ID (UDID) when stuck halfway through the iOS 4.2 beta 2 update

Yesterday's update says it should only be installed on a device used for development testing.  Okay, that's what it always says. No problem...  

But, if you hadn't specifically registered the device ID in your dev account, then after wipe and install, iTunes blocks saying the device isn't known.  Problem...

Unfortunately, the usual method of getting the device ID by option-clicking on the device info screen in iTunes won't work, because iTunes is stuck refusing to activate and access the device.

Go to Library/Application Support/MobileSync/Backup, where your iOS device backups are in folders with 40 character long names.  These 40 hex character names are your device IDs.  Copy the folder name that's the backup of this device, and paste it as a new device ID in your Apple Developer device ID control panel.

Unplug and replug your device, iTunes will activate it, and you'll be offered to restore your backup as usual.
2 responses
Thanks Michael, this just saved me a lot of time when running into the same issue with iOS 5 SDK.
Thank you so much!! In OS X Mountain Lion the path its: /Users//Library/Application Support/MobileSync/ Also, you can find the corresponding backup un finder through iTunes in: Preferences, Devices tab, right-click on the backup from the device and select 'Show in finder'. I suppose this alternative should work on Windows too. Note: Watch out, if you have more than one backup for the device, you will have to trim the backup date from the folder name (from the beginning to the first dash!)