I finally installed Antix on that same disk in an USB enclosure, from my recent Dell XPS13 booted with an USB stick.
Guess what, same old grub recue prompt for the Toshiba Tecra.

I then dug up a slightly less ancient Thinkpad X30 I had forgotten about, dropped that hard disk in it, and, guess again, Antix 16.1 booted just fine.

Which to me means there is something in the BIOS or the ATA interface that Grub does not understand? Remember this Tecra boots Windows 2000 in that same disk.

Anyway, I'll see if this X30 is any good, in which case it has got a new lease on life.

Thanks to those who helped me !