^ If you are very concerned of loosing data and really want a secure and reliable setup, then a single drive config would not be a very good option...
I would suggest RAID 1 with 2 similar drives as they are more reliable because when 1 drive fails, the data is still safe in the other...
But if you are not willing to RAID, then for backup drives, WD Green series drives are very good options...
RAID does not a backup make. RAID 1 or 5 protect against a drive failure, but that's only one of a very long list of possible things that can cause your data to disappear. Others include viruses, corruption, accidental deletion, theft, power surges, etc. etc.
If your data is important then you need to copy it to offline storage of some sort, and for the best security there should be two offline copies, one of which is stored offsite. It doesn't matter that much what you copy the data to, be it tape, USB disks, DVDs, or an Internet storage provider. The important thing is: two offline copies, one of which is NOT in your house.
WD Green drives are decent choices for backups, but having TWO offline copies is MUCH more important than worrying about the small reliability differences between various brands. Even if you buy two of the least reliable disks they're very unlikely to both fail at the same time that your online drive fails, which is what it would take for you to loose your data.
I don't know how important Your data is and how much of that 500 GB is the really important. I don't count music or movies as important, it is the irreplacable things like family photos and videos and documents/pojects that are important and valuable which would take lot of time, effort and money to recreate.
While having local backup is convienent and fast for really important data You should be looking to backup really important data in different geographical locations or online storage at least 2 different places.
For local backup external HDD which is connected to PC only for backup/restore is better as if PSU fails it might take out all Your internal HDD's as well. Even better if You have enclosure with RAID 0 with 2HDD's.
If the reason you're buying a disk is to have a 2nd copy for backup (and that's a good reason!) then you should DEFINITELY get an external disk that you copy your important files to and then store separately from the computer. And as I mentioned the very best safeguard is to actually have your data stored in TWO places outside your machine, one of which is in an entirely different location (in your desk at work, for example).