Which WD HD Should Buy?

Red is "intellipower" - variable rpm (max around 5k I beleive) drives for NAS boxes and the like, budget 24/7 drive with lower warranty and performance.

Black are enterprise class - high rpm and 5 year warranty, I use them in HP Proliant MicroServers all the time.

Blue are the slim desktop ones I think - I don't really use 'em.

Google "WD Blue", "WD Black" and "WD Red" takes you to the WD website descriptions - not hard to find!