ever since i worked as onsite tech support at law firms for 8 years, ive always considered hard drives as temporary storage. anyone that thinks storing backups on hard drives is safe is deluding themselves. even RAID isnt a completely safe bet - in several cases i have seen both drives in mirrored RAID-1 arrays fail at the same time, resulting in a complete loss of data. and while it is safer, most home users dont use RAID.

DVDs are the safest bets. you can pick up a DVD burner for $40-50, and DVDs are dirt cheap. you could back up your entire poser runtime each month onto a DVD or two for a quarter, and if somehow you damaged the DVD at most you would have to redownload a month's worth of purchases. stored properly, DVDs and CDs will theoretically last between 80-150 years. a harddrive is a ticking time bomb.

an external harddrive can be convenient if you are using poser on several computers, travel a lot, are using it on a laptop, or move other data as well. there is no need to buy retail 'external harddrives' (not to mention that LaCie is pretty expensive). you can simply pick up an external enclosure like this one and put in any internal harddrive you like. there are hundreds of types of external enclosures ranging from $10-100, i simply picked that one because it has both SATA and IDE support. if you have a MAC then i would recommend getting one with both firewire and USB 2.0.
(also, if you have an old external hdd that 'died', its highly likely you can just reuse the enclosure, although many manufacturers make them so they cant be opened/reused).
for IDE harddrives, ive had good experiences in the last few years with maxtor - they make cheap, reliable, high capacity 7200rpm drives with good read speeds, 16 meg cache. i will not buy western digital anymore, and fujitsu has always made the worst POS drives on earth. lacie uses maxtor drives, if memory serves.

last month, one of rio's girlfriends knocked my external harddrive off my desk onto a tile floor. with harddrives you not only have to worry about mechanical failure, and inevitable wear, but also accidental damage since they are incredibly fragile and shock sensitive. backing up onto DVDs is a safer, cheaper and more permanent backup.