I've run across at least one system where a second installed drive wasn't connected--probably a dupe placed in the box in case the primary failed.
Laughably, a place I worked at had 1.44 meg 3.5" in 5.25" bay fillers on a bunch of AT class and a handful of 386 machines, and being late 1990's nobody really ever tried to use the 5.25" -- it was a laugh they were still using 286's but they got the job done. (Word 6.0 for DOS, Procomm into a *nix mainframe)
I had been there about three months when I got a call that the secondary drives weren't working in the ENTIRE marketing department... so I go down there and pop them open, NONE of the drives were connected -- the cables didn't even have the edge-card connect for them! Looking closer, they were ALL 360k drives...
It turns out the previous IT guy was recycling AT cases on upgrades and didn't have the proper blockoff plates, so he just slapped the 360's in there as the boss didn't like the gaping holes...
Sucktastic part was the IDE hard drives were double sided taped into the cases either on top of the PSU or packed in tight under those floppy drives. HERPADERP.
I think it was that job where I started muttering "sleazeball shits" under my breath a bit too often.
Same place where most of the employees preferred using the VT-100 knockoffs (I think they were WYSE branded?) as those were connected at 192000 baud stable, while the PC's could barely manage a quarter that reliably. I ended up building an alloy slave network to let four of those terminals run Word remotely and have access to the membership and development databases. (the latter written in Paradox). Confused the hell out of them though when they wanted to make a copy and had to use a floppy drive in another building to do it.