I've never done this before, and I'm fairly inexperienced with computers--sorry, but please try to bear with me. I was thinking of purchasing three internal 3.5" HDDs (1TB) and putting them in RAID 5, so I get 2TB of storage space and one drive can safely fail before I lose any data.
Basically, my question is--how do I go about doing this? I want to purchase a 3-bay enclosure that supports 3.5" drives, I think, so what enclosure should I purchase? I've looked all over for information and recommendations on enclosures, but I just can't find any sort of definitive source or answer! Looking for a USB 2.0 interface and RAID 5 support. USB 3.0 or eSATA is a plus!
RAID isn't a backup. It protects against drive failure, but it doesn't protect against a whole host of other risks such as viruses, corruption, accidental deletion, power surges that damage equipment, theft, etc. etc. And RAID actually adds some risks (such as screwing things up when a drive fails and you have to replace it - more common than you'd think).
If you're looking to protect precious data, then you need to get a backup strategy in place before you worry about RAID. A good backup strategy puts copies of your data onto external disks that are only connected for the purposes of running the backup. Ideally you'd have two or more external drives that you alternate, and one of them would be stored offsite.
Once you've got a sound backup plan in place, then you can look at RAID in it's proper context - to reduce downtime due to failed drives. THAT's what RAID buys you - less downtime. It doesn't do a very good job of protecting your data.
Sorry - I can't recommend any enclosures specifically, but I'd strongly advise against USB 2.0. It has a maximum real-world transfer rate of around 35MByte/sec, which will be a real bottleneck for any modern drive.
You should also be aware that some people have had issues using standard consumer hard drives in redundant RAID arrays because they don't have TLER (Time-Limited Error Recovery) - this can cause them to be mistakenly declared as "dead" by the RAID controller. "RAID-Ready" drives have this feature and tend to give you a more robust RAID subsystem.
The last piece of advice I have is to spend some time familiarizing yourself with the operation of your RAID array - in particular you should test your failure recovery procedures before you commit live data to the array. If you're depending on the array to keep trucking when a drive fails, the last thing you want to be doing is experimenting on how to recover when a drive actually does die on you. It's at this point that a lot of casual users seem to end up loosing data.
+1 on what Sminlal has said about backup. I would have said the same thing but Sminlal saw your post first. A good example of what raid is used for: pick any online retailer. They need their product catalog and checkout software always running. So each runs on a raid array. If one drive dies the catalog/checkout software is still available to customers at a reduced performance rate, ie slower. If it died completely the retailer wouldn't be able to sell it's products and thus lose revenue. That is what raid is for, to reduce or eliminate downtime thus keeping the data or application always available. Why people equate raid with backup I'll never understand as it exposes your backup to unecessary risk.