Actually, without being an expert on topic, similar things happened to me in the past, and there tends to be an "easy" fix to the problem, at least so you can disk mount and disk-utilitize it properly (then a repair disk works for me): just leave the usb disk plugged in. A loong time. Like 30 minutes. For me, three times at least (two different drives), the disk ended up mounting. Each time indeed, one of the partition was damaged, but could be repaired.

But disk warrior is indeed great software, it saved me from windows destroying a firewire drive repeatedly (odd problem: the firewire drive mounts and works perfectly in mac os, but if i put in windows, it will mount and work fine, but refuse to dismount, though i am not accessing it. then if i just turn off the computer, the proper way, at reboot the partition table of the drive is fucked. odd to say the least)