This sounds like a small NAS device meant for RAID1 specifically (data mirroring for reliability). Most are 3.5" but I imagine there's 2.5" ones. I suspect the cost is high and that you should stick with a single USB drive deal.
Single 2.5" USB:
2.5" USB drives do not require power. You can simply buy a 2.5" USB case. You may wish to get one compatible with eSATA but that's not really a big deal and adds to the price.
USB drives are now quite cheap now and a new USB hard drive might not cost much more than a USB case.
Go to this online store: www.ncix.com
I use Acronis True Image to backup my Windows drive to a USB hard drive. As long as you have an internal or USB hard drive detected the free version from Western Digital will work.