I don't know what the problem may be but, may I suggest that you attach a picture showing how it's all connected?

Did you set correct 5.25" drive specs in BIOS? I had a similar problems in XP with 5.25" that was giving me errors. Found out that I set drive specs wrong.

It's just a standard 4-pin power connector and FDD ribbon connector. The 5.25" drive is daisy chained to the 3.5" drive.

Try connecting the drive jist by itself before or after the cable twist. See if any of those works. Also yes, ensure in the BIOS or the motherboard's DIP switches that your drive setup is properly declared.

Only additional I can really offer without asking questions that will likely only cause confusion--

Make sure that drive A comes after the twist, and that drive B comes before the twist.

If things are still acting wonky after verifying that, THEN we can start asking the really confusing questions.

I disconnected the 3.5" drive from the ribbon cable, rebooted, and set the 5.25" drive from 360 KB to 1.2 MB in BIOS.

Still no success. The LED lights, the drive whirls for a good 10 seconds, then Not ready reading drive B:

An observation - BIOS keeps resetting the 5.25" drive to 360 KB despite my setting it to 1.2MB. Which is fine, that's
how I need to use it. But it seems odd it won't set to 1.2 MB.

OK, confusing questions time.

On the 5.25" drive, are there any small jumpers like these ones?

Some 5.25" drives are "Fully Shugart compatible", meaning they can be configured with a cabling ID for an untwisted cable, with support for up to 4 drives. They can work in a variety of other computing platforms, like TI systems, or Amiga systems, for example.

The IBM PC has a kind of clever hack going on with the twist in the ribbon cable. It essentially swaps some IDs and motor select lines around, so that users dont have to fiddle with jumpers (and in later eras, such jumpers vanished altogether)

If however, your drive is configured to be "Drive 3" or "Drive 4", then strange things will happen with the cable twist.

The Toshiba 5451 is a 360k drive, not a 1.2Mb. The jumpers are in one of the set of docs I upped to Vogons Library:
http://www.vogonsdrivers.com/getfile.php?file … &menustate=29,0
to make it easy here is the basics. The picture show typical PC usage. If the drive is connected before the twist and a 3.5 is at end of cable. pull the Term jumper off, only the end cable drive should be terminated.....

Thanks, that is good info. It does not look like TM is set on the Toshiba drive:
[unset]
[unset]
[set]
[set]
[unset]
[unset]
[unset]
[set]
[unset]

If the 3.5" drive works I would suspect the drives. Start with the basics, did you clean the heads? Does the 5.25" drive work in another PC? Wouldn't be unusual to find a bad one -- 360K drives are getting pretty old.

Good questions. At present I don't have any other system to test the drives on. And the PSU on my 386 has gone south. There will be a slight pause while I track down another PSU with the the appropriate 386 mobo connector.