I got both the WDs working. Made sure both were in Master (one already was, one did have a jumper which I removed to set to Master). Then ran these commands in Terminal

diskutil list

to get the disk ID

sudo gpt recover /dev/disk(#)

where (#) was the disk ID

Both drives then mounted.

The old IBM is a different matter. I set jumpers to both "Master 16 heads" and "Master 15 heads" (don't ask me what this means) but none of my machines can even see it. BTW one of the machines is a non-intel G4 mini. Judging by the occasional click I hear, this old drive didn't like being removed and is kaput. I don't know what else to try—any suggestions are welcome. Naturally it is the one containing the last bit of unbacked-up work I did.

Got a new (used, new to me) MacPro box headed here Friday and the weekend to try to get things back up in something resembling how things ran on the G4. There is some legacy software that I am little concerned about, but will load Snow Leopard and with luck it should run under Rosetta.