Hard disks are more reliable than DVD. Unless you are willing to pay a premium for premium made Optical media the bio-dyes and layer binding glues deteriorate dramattically over time and can destory the disks.
Smaller hard disks are cheaper, faster and longer lived. - just make sure to run something like GRC spinrite occasionally over the disks to read/write-back the data to counteract the earths tendency to erase magnetic media over time.
Larger disks though convenient are more prone to substantial data loss.
The plan was to burn DVDs to watch and then keep a backup on a HD for when the DVDs get flaky. The HD would be external and only be connected when I wanted to clear off my regular hard disk.
A few percent of my saved movies become unreadable after a while and a few more got scratched. There has been no pattern with media brand, just a few that randomly go bad after a few months. Some of those I lost were good ones and I don't feel like paying for them again. Since HD long term storage is highly reliable I felt it was better than burning 2 sets of the DVD, though that is an option.
Eventually I will get off my lazy butt and put together a multimedia server but for now I'm looking cheap and fast. I can do a 1T external for $120. Also, I do not like wasting the electricity an always on server consumes. Color me Green.
Mainly I was looking for feed back on the reliability of BIG, non-redundant hard drives. I have never had an issue with a drive under 160gig failing.