I've just hit exactly this problem over the weekend where my WS Elements 2TB USB drive has completely failed on me. I _just_ have enough space to create a new backup on an internal drive but I'm getting antsy until its replacement arrives.
I've only had problems with WD drives but that's because I keep buying them based on the reliability of their WD Passport portable drives. I'm on my third Passport now and never had a problem, but I've had return-to-manufacturer level problems with two WD My Books and now the WD Elements.
I'd just, independently, come to the same conclusions listed above which is to buy a second equivalent capacity disk from a different manufacturer and alternately cycle the two backup disks.