Are you looking for drives that will mirror each other?
If so, I bought the G-SAFE system about 4-5 years ago. 2x 3TB drives in one housing which use USB to plug into my iMac and mirror each other. I use Good Sync software to sync my LR photo collection to the drives - which automatically mirror.
I also have a third G-SAFE 3TB drive which I rotate in and out of the G-SAFE housing with the other two drives so having 3 copies plus the original on my iMac. The third copy is kept "off-site" in a Pelican case!
I have been really pleased with this system and feel very confident in its backup capabilities. I think, at the time, it cost about £400 and it has proved very reliable.
G-Technology is the manufacturer and there is now an updated system.