Mountain Lion won't mount backup drive

I recently upgraded to Mountain Lion from Lion.

For some reason, the upgrade did not go well, so I went to the Genius Bar.

Genius bar man asked if I had backed up my data, I said yes, in fact just that morning.

He wiped my drive clean and reinstalled Mountain Lion.

Ever since then, I cannot access my backup LaCie drive.

It does not show up on the desktop.

Any ideas??? Disk utility says both hard drives are OK.