That was surprisingly unrantworthy. Restored the previous disk to the new one and erased. Set the now blank old drive as the Time Machine drive. Now slowly backing up 204 GB of, what, I'm not sure. Well, I'm sure it's not what you lot (or my wife) thinks.
The dead drive is apparently alive. I think the issue is that it, for some reason, doesn't correctly unmount when the machine sleeps and if it's halfway through a backup, then it corrupts (always restarts with a 'disk didn't correctly...'). Inexplicably (to me), the other USB drive (and now TM drive) always unmounted properly when the machine goes to sleep. It wouldn't seem to much to ask, given one of the more convenient features of a Mac is the instant sleep and wake (did they ever figure that out for Windows?)
Not sure what I need another half terabyte of storage for. To the understairs cupboard of IT shame and spaghetti-esque wiring it goes.