Well the ext HD was never set as the boot volume (I've read about doing that, but can't remember how and didn't in this case) and it was hooked into my Ti-PbG4 via FireWire as per usual. Weird.
But that's "bygones" as they say. Esp. since that volume as it once existed is now erased and getting restored-onto by Disk Utility, from the SD-produced backup copy of a couple days back, that I was using for a few hours while copying that selfsame image onto the internal HD.
The important question (that I will have an answer to in a couple-plus hours) is "will the internal HD be bootable after being restored properly (after having booted from the Panther CD) onto from the ext HD, despite my having booted from that ext HD and done the first restore from it and otherwise used it for some hours earlier?" I'm gathering, from your answer, that it'll be OK, and that your answer to (2) is "No, there's no reason to expect that it'll be unbootable."
Film at 11!
OBTW
thanks again.