I always recommend tape backup,
OR a
RAID 1 enclosure with 2 mirrored disks, each from a different manufacture hooked up to a power flux absorbing UPS [
uninterruptable power supply]; as power fluxuations are a big cause of hard drive problems. I know a guy that has had external drives powered up most of the time for the last 6 years with no problems because he has them hooked up to a UPS for very stable power supply.
If you only backup to a single external drive, then delete the originals from your computers internal hard disk, you're asking for a lot of problems.
Hitachi hard drives are very good BTW. I always recommend those. Seagates and Maxtors are the worse.