[freakshow]

for your parents, just use dropbox/google drive/sky drive or anyone of those cloud services. It's worth paying a few bucks a month to save the headache of having to recover data.

There's no way, ever, that they'll remember to hook up their external drive, or even want to copy files to a NAS. And a NAS will require (your) maintenance as well. It may not even be cheaper after buying the drives, replacement drives and NAS box/computer.

The only caveat to this would be if your parents store over 1TB of data(photos/videos)… then there are no cheap/easy solutions.