Did a little digging since I've been considering improving my backup situation- it looks like the DVR boxes out there are simply hard drives with software for talking to your digital video recorder. Format these puppies and you should be good to go. However, I didn't find a shortage of straight-up eSATA drives out there... wouldn't it be simpler just to grab of one those (so you know 110% it'll work)? The concept of eSATA intrigues me (fast speed, low CPU usage), however for my purposes I think I'll be sticking to my original plan of getting a NAS.
Since I like the idea of not having to pack it with me everywhere I go (even tho sacrificing some speed to do so), RAID redudancy, able to FTP in, as well as a built in Torrent app and media center (to play nice with xBox 360, PS3, etc)- I think I'll be snapping one up any day now
Let me know how your eSATA adventure pans out! I'll do likewise.