Thanks to both.
This is turning out to be a BIOS/partitioning/drive parameter identification problem and may be unsolvable without re-installing NT4 on the drive in the newer PC.
PC Alpha is a 1996-7 HP Pentium with Phoenix BIOS Rel 4.01 and sees the drive in "Auto" the way it should. NT4 loads like a box of snails rather than a box of birds (it's a 166MHz!), bit it's as solid as a rock. Partition Magic sees it exactly the way it should. The disk was created three years ago and funtions as well as it can at that speed.
PC Beta is a 1999 Dell Pentium 500MHz also with Phoenix BIOS Rel 4.5 and sees the drive in "Auto" the way it should. During Boot, it finishes the POST and stops with the cursor flashing in the top left corner of a black screen; that's it, not even the dreaded "operating system not found".
When PC Beta is booted with its regular Win2K disk, and the problem HD is installed as slave, the BIOS sees the drive in "Auto" excatly as it should. Windows Explorer sees the drive too and I have copied off the Data that I want to hang on to.
Partition Magic, however, now doesn't like the drive and calls it bad; it sees a different set of Cylinders (240 vs 255) from the manufacturer's figures. So this is the boot problem, but there is no way to force PC Beta to see the disk in the same way that PC Alpha does.
I've tried back loading a BIOS flash to update 4 as opposed to update 11 but it made no difference.
So next step is to re-partition in PC Beta and re-install NT4. It'll be interesting to see if PC Alpha will see the drive then.
I passes the time...