I've killed perhaps 50 external portable (mostly USB, but also firewire) drives over the years. We have very close to a 100% failure rate. They are fabulous little things (and getting better all the time) for what they are, but suitable archival backup they are not.
For a small personal system, it's still pretty hard to beat tape.
Solid state memory might be worth checking out. I don't know much about it, other than my cheapo keyring thing has been in saltwater, my nasty pocket, the washer, the dryer, and who knows where else for around 5 years and just keeps on working.
Depending on what your 500G is and how you manage it, you may be able to push incremental backups. If you're dealing with textual data, unless you're the US Census Bureau or something, I would suspect huge inefficiencies/redundancies somewhere in the pile - it might be that you can clean up to a more manageable bundle.
Disk-based systems (including/especially most RAID setups) rely on someone noticing that part is broken before the other parts
break. That's usually not happening in a personal backup system.
For work, we use massively redundant disk (about 5PB worth) and a mirrored (on 2 continental plates) 3 exabyte robot-controlled tape library for incremental backups. I really wouldn't trust much less for irreplaceable data. (And it only cost a few hundred million dollars to build!) I really don't know much about them, but I suspect some of the better online backup services offer that class of protection for a few dollars per month - if I had personal data worth protecting, I'd seriously check into them.
For my actual personal junk, I use USB drives and time machine (for now) to occasionally back them up to each other. Failsafe? Not even close, but sufficient for what's on them. My pictures are on Flickr, but Yahoo has been making me nervous lately, and maybe it's time to starting thinking about a new home for those.