Switched (cloned) to new hard drive; only boots w/ old one plugged in

But if the boot process gets disrupted, that would suggest a bootmgr problem - wouldn't it.

Fair enough - I may be / hopefully am wrong. But I used that tool to move the boot manager from C (just going to use that to refer to "old drive") to D ("new drive"). Now, when both drives are in, telling the BIOS to boot D results in the OS on C being booted. And when C is removed, D gives a good old-fashioned boot error (because the boot manager is now referring to a drive that isn't there).

I'll try repairing the boot manager again with the RE. I'm expecting the best-case scenario with that is going back to the type of error I was getting before, but maybe it'll be different.