re: What kind of External hard drive should I buy for backup?

I've got a drobo. You can't get them in in many different configurations. I've only got the 4 bay. It's nice because of redundancy. I've got 4 4tb drives in there so it gives me about 8tb of space for backup. If one drive fails or starts to fail it has a warning light next to that particular drive. You push the eject button and swap it with a new one all on the fly. I've had it for four years and had to replace two drives in that time.

Had a 2tb my book before that that failed and I had to pay someone to get my stuff off of it.