Re: USB HD is there but not mounting
my guess is its not being recognized or is not reporting itself to the os. Have you tried rebooting with the drive plugged in? Also, look in your log files under /var/log/ for any reference to the hard drive. The dmesg, syslog, kernel.log could be worth a look.
Beyond that, I really have no other suggestions other that trying it on another computer to see if its even working. If it fails to mount on another computer, then I'm afraid you may have a dead hard drive. Sorry.
My PC: ASUS P5QC Motherboard w/ Intel Core2Quad 2.83GHz processor - 8GB of DDR2 800 RAM - nVidia GTX 550 TI 1GB - 1 500GB WD SATA Hard Drive - 750GB WD SATA Hard Drive - 2 ASUS DVD+/-RW DL Burners w/ Lightscribe