At the Apple Icon at top left>About this Mac.
Then click on More Info>Hardware>USB, does it show up there?
No idea why Mobile me is popping up.
"Try Disk Utility
1. Insert the Mac OS X Install disc that came with your computer, then restart the computer while holding the C key.
2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu. (In Mac OS X 10.4 or later, you must select your language first.)
Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.
3. Click the First Aid tab.
4. Select your Mac OS X volume.
5. Click Repair. Disk Utility checks and repairs the disk."
http://docs.info.apple.com/article.html?artnum=106214
Does the external show up there?