Solved. I changed the location of the master/slave/cable select jumper on the back of the HDD from master to cable select. Now, the HDD functions correctly, without the requirement to include a slave optical drive on the ribbon cable.

One thing bothers me though - I was going to use this HDD on an ancient 486 mobo. That mobo may not be able to cope with a slave optical unit, and so I will have to leave the HDD jumper on the cable select setting. But will the cable select setting work OK using an old 40-pin ribbon cable?

Some drives have jumpers for master/slave/single.

Doesn't it have another setting? Most have something like "master (slave present)" and "master (no slave)", possibly by removing the jumper altogether.

Where would I be without you guys! 😁 Removing the HDD jumper gives me the setting "single master", and this works just great!

Don't you just love playing games with ATA interfaces? With all the quirks you find its one of the reason I like SCSI so much, at least there all you really have to worry about are Device ID's and termination.

Yeah, they didn't have cable select (CS) for old-school ATA/IDE devices. A common array of jumper settings was:
- Master (w/slave on other connector)
- Slave (w/master on other connector)
- Single (no drive on other connector)

Gotta love those old 50-wire SCSI computer bowels 😉

My first PC compatible had a 40MB MFM HDD. It was the size of two 5.25" HDDs stacked on top of each other. My dad eventually took it apart long after it was obsolete to show us the inside of a HDD in person.

WD is AFAIK the only one that used different settings for master w/ slave present and master single. For a long time I thought my hdd was damaged and only worked when slave was present 😁