Any idea why the primary interface probe lists the Toshiba six times? Could this be indicative of a problem, just on the edge of not working...?

No. The kernel essentially asks the IDE bus "Is anybody out there?" Each attached drive replies. It has to ask several times, as some drives take longer than others to get themselves sorted out, and do not respond to requests until that time. Once a certain number of requests have resulted in only one reply, the kernel will decide that there really is only one drive out there.

Note to FAQ/empeg folks: It would appear that some (only this model?) hard disks might appear to not work after disk builder alone. Proceeding to the upgrade process _anyway_ worked in my case.

That's the way it always works. Strictly speaking, the error message is misleading: it's actually "No mountable root partition found", but the leading cause of that is there being no disk at all. I guess these two errors should be kept separate, to avoid confusion for anyone who has applied a disk builder but not yet an upgrade.

Peter