I am thinking maybe elaboration may be important here:
First, calling a HDD a backup, esp. versus traditional backup technologies like tape has several issues. The following considerations are not exhaustive, but things to think about if you value your data:
1. A backup should be removable and the media should never return, except in the case of a restore. "How far" is debatable, and has a lot to do with the nature of the data: other side of the room, other part of the building, off site location, out of state, other continent in an old salt mine, etc. The basic notion is that the backup should not be vulnerable to the immediate physical and software/system threats to that which is being backed up.
2. A backup should be of a media type different than the source media. This is due to the reduction of possibility of the same issues affecting the both source and backup similarly. Again, both physical and system-wide. A virus that wipes your source HDD will probably also do the same to a 1394HDD on your system, but will probably not affect a tape drive.
3. Tape media are much simpler physically/mechanically than drives. If the case does go bad, the tape inside is usually still good and can be re-spooled easily and with a high degree of success, compared to issues of head crash, motor issues, etc. that can plague drives.
4. In spite of the reduction in HDD costs, tape still tends to be much less expensive. Both go down in price all the time. Comparing my current tape backup set of about 8T the cost in today's $ is about $6300.00, including the tape drive, or about $0.80 a G. An external drive solution, assuming about $250 for a caddy-style enclosure, about $225 for 250G drives, and another $50 per drive for the caddies, is over $9K. Think in terms of at least three or four years when considering backup costs.