It turns out that the 'failure' of my backup HD was not hardware but software. While Disk Utility couldn't deal with it in the state it was in, by reformatting the HD via a PC running Windows, then reformatting that via the Mac, I got a functional HD back. It now has 3 partitions for various backups, including bootable clones, Recovery HD, and other stuff..
It's somewhat sad that I've noticed an increasing number of "problem child" drives that I've had to resort to alternative methods to format because Disk Utility was being stubborn.
I can count maybe four times in the last year I've had to drop into terminal and (
caaaaarefully) use DD to drop some zeros on the starting few MB of a device to get Disk Utility to finally see it as "unformatted" and allow me to (re)format it.
