Rocstor is a great customer-service oriented company and Arcticroc a great plug-'n-play, Mac-style product. I had an external Rocstor RAID that I purchased in 2008, which failed after the warranty. Despite the warranty being out, Rocstor replaced it for free with one of these Arcticroc units (a 1TB variety). I have since replaced the dual 500GB drives with dual 2TB drives, and purchased an additional 4TB Arcticroc for 8TB total external storage.
The drives are easy to swap out. You just pop the top cover off the unit, slide out the drives, and then pop in new drives. If you don't have an extra set of rails, then you have to switch the rails to the new drives, which involves 4 screws (not hard). So I would suggest picking up an extra set of drive rails (you can order them from Rocstor). That makes it easy to have a backup set of drives that you keep in a safe deposit box in case of fire/theft.
I've had it in use for Time Machine on my Mac Pro and it has worked like a champ for months and months. Don't forget to also use a UPS power supply!
Recently, I've bought this RAID system that holds two 2TB HDDs and it works great! Because it has 4 types of interfaces, I don't have to worry about which interfaces my host systems have or anything. I have both a MacBook Pro, Asus laptop, PC desktop, and mobile 2.5 HDDs. Sometimes I use eSATA to connect between the RAID system & Asus or PC. Other times I use Firewire to connect between the RAID system & My Mac. Of course, the USB 2.0 is universal & in all my mobile HDDs, so I love it! Plus the RAID management is easily to set & oversee. This is definitely not one of those cheap mock storage systems!