This is all great info. My NAS is running out of room (46 gig free of 3.5TB) and I'm looking into 2TB drives as well. I've learned a lot reading this thread and I thank you all for that. I've currently got 5 WD10EADS drives and was looking at the WD20EADS as replacements. However, I think I'll go with the WD20EARS based on this thread.

How do I figure out how much my usable space will increase by swapping one of my 1TB drives out for a 2TB? Basically, I'd like to calculate how large of a timespan I can spread my 5 drive upgrade over. I think it's a RAID 5 configuration.