Unfortunately, the WinXP CD's Repair option is available only when the
CD can see the hard drive. I used it to run CHKDSK, as noted above,
but now it is not seeing the drive at all.

In the big computer, I tried making it both primary and secondary IDE
master. In both cases, I tried both master and cable select jumper
options. In no case did the machine recognize it. The machine did
boot as usual when I used the usual hard drive as primary master.

The partitions are mostly NTFS. One is FAT32.

I've heard good things about SpinRite over the years. But I suspect
it, too, will not see the drive. When the drive is nonfunctional, not
even the BIOS sees it.