a external harddrive is just as likely to fail as the internal drive on your home pc.
actually i see some die more in external bays more often than your pc. I've seen the actual external "bay" go bad. take the drive out of the bay and the data is still there.
i personally don't use them....unless you need mass store age
a DVD-RW drive is your best bet. along with nero 7, which has a automatic backup program, works flawlessly, as long as you leave the dvd in the drive
if i were to get a backup drive though it would be seagate