I have a MacBook Pro. I normally back up using Time Machine and the backup is saved to an External 2TB Seagate FreeAgent drive. That drive was about to die on me, so Seagate sent me an identical replacement. To give me extra backup security, I also purchased a 3TB Seagate Backup Plus drive. I copied everything that was on the 2TB drive to the 3TB drive, including my Time Machine backups, so those two disk were identical.
I was trying to clean up the MacBook Pro's HD using TechTool Pro, but to optimize the HD I had to clone it to the 2TB external drive. I failed to wiped that 2TB drive of the Time Machine backups on it prior to cloning the HD. So when I tried to clone the drive back to my MacBook Pro, I couldn't, because the material on the external drive exceeded the space on the MacBook Pro HD (i.e., HD plus Time Machine backups). However, before I realized that I couldn't just clone the HD back, I wiped the HD on the MacBook Pro.
Long story short, I ended up reinstalling my original MacOS onto the MacBook Pro, then installed the upgrade from the install disk I had. I wanted to restore my MacBook Pro from my Time Machine backups, but I get an error using Migration assistant: You can't transfer from your other Mac to this one because your other Mac uses a newer version of Mac OS X. You must upgrade this Mac before you can transfer from your other Mac. The Mac OS currently installed is Snow Leopard (10.6.8), even though I had purchased and upgraded my MacBook Pro first to Lion (10.7) and about a month ago to Mountain Lion (10.8.2).
It seems like I am in a Catch 22: I can't restore my HD from Time Machine, because it says I need to install Mountain Lion OS. However, I can't do that since I don't have a startup DVD of Mac OS 10.7—I downloaded that directly from Apple's Web site.
If I could find a way to remove the Time Machine backups from my 2TB drive, I could simply clone the HD back onto my MacBook Pro's hard drive. Shy of that, I had hoped to be able to restore my HD using the latest Time Machine backup I have, then start the clean up and cloning process all over again.
I welcome any suggestions for overcoming these problems.