My C:\Users\Scott\AppData\Roaming\Apple Computer\MobileSync was almost 25 gigs and on a 256 gig hard drive, that's 10% and that's tight. I wanted to move it but there's no officially support way.
Here's a way. It's totally not supported and could totally screw up your computer, so you've been warned. Remember that you googled your way to this blog and I'm just a random guy. No warranty, Dear Reader. If you are reasonably savvy and you understand that this Works On My Machine, then we'll get along fine.
Here's the idea. You'll move it to a drive with more space, but you'll LIE to iTunes using a little-used Windows Utility that will make a LINK between the folder iTunes expects to find and the folder you want your backups in.
mklink /J "C:\Users\yourname\AppData\Roaming\Apple Computer\MobileSync\Backup" "f:\yournewfolder\Backup"
You should see something like this in Windows 7. Note the little "shortcut" overlay icon? That's saying this is a link.
You can also confirm it with dir from the command line. Note the <junction>:
C:\Users\Scott\AppData\Roaming\Apple Computer\MobileSync>dir Directory of C:\Users\Scott\AppData\Roaming\Apple Computer\MobileSync11/25/2011 10:10 PM <DIR> .11/25/2011 10:10 PM <DIR> ..11/25/2011 10:10 PM <JUNCTION> Backup [f:\iTunesMobileSync\Backup] 0 File(s) 0 bytes 3 Dir(s) 97,594,851,328 bytes free
If you are still on XP and not Vista or Windows 7, you don't have mklink, but you can use the junction utility in the same way.
Again, if these instructions don't make sense do you, I urge you to find a techie and please, be careful. You've been warned. That said, I've just opened up 25 gigs on my C: drive, so I'm happy.
Scott Hanselman is a former professor, former Chief Architect in finance, now speaker, consultant, father, diabetic, and Microsoft employee. I am a failed stand-up comic, a cornrower, and a book author.
Disclaimer: The opinions expressed herein are my own personal opinions and do not represent my employer's view in any way.