I pulled out the connection with the coloured wires before reinstalling windows, and pushed it back in after. Didn't touch the ribbon connector. (I have a horrible suspecion maybe I should have disconnected both...)

I'm using Windows 98. And no, the drive doesn't show up in Control Panel/System.

I haven't tried exiting to DOS - I'll have a go at that after I've posted this. (I'm not at all good at making sense of DOS.) But as I said, from the stuff that flashes up on screen in black and white while the computer is starting up it seems that the computer does know there are two drives.

On advice of the hotline bloke, I started up with the second drive disconnected, and at one point during the opening process it reported that the slave drive was gone, and I had to push F1 before I could continue - and then when I connected the drive up it stopped doing that.

Thanks folks. Keep coming back with pointers.