Can I move Mobile Sync folder to another HD Can I move my MobileSync folder to another HD on my iMac to free up space on my startup HD?
You can move the folder to another drive, then use a symbolic link back to the original location, so iTunes can still find it.

A symbolic link is a bit like a shortcut or alias, that the system can use to target a file or folder in another location.
Because of the rather arcane method you'd need to employ in Terminal to generate them, I've always used a little freeware app that sets itself up as a Service in OS X, meaning you can access it through the right-click menu - SymbolicLinker

After Installing this [reboot may be required if it doesn't immediately show in the right-click menu], the procedure would be  


Move your MobileSync folder to your new drive, Cmd/drag will move rather than copy.  
Right-click the folder in its new location, go to Services at the bottom of the menu, then Make Symbolic Link  
Cmd/drag the created link back to the original location in ~Library/Application Support/  
Change its name to remove the 'symlink' at the end, so it has the original name MobilSync  
Done.