Bought 2 Seagate Goflexi 320gb portable drives just over a year ago and one failed on me 2 nights ago. One minute worked next didn't..... And the other drive playing up.... Just be aware that what ever you get, buy 2 just in case one crashes, unfortunately, these backup drives don't seem as reliable as you computer hard disks.
Western Digital seems to have a good name, but then again, Seagate were the daddie when I bought mine. At the end of the day, its pot luck, but the USB portable drives do seem very prone to failing after a short period of time, 1-2 years from the reading I've done, but would recommend buying two devices, one to back up the other.