What IDE/UltraDMA transfer rates/modes does the controller support? Have you tried using an 80-pin IDE cable?

Also, make sure the jumpers are set on the drives for master and slave (not "cable select" or "single drive") and try swapping which IDE cable connector is connected to which drive.

That controller is a Promise DC-2000 VL. From the readme, it reminds me of my old Promise EIDE2300 in some ways but seems to have fewer speed tweaks. My guess is that it supports PIO Mode 3 speed levels. You can enable read/write multiple and VL 32-bit I/O too with the driver. These features may or may not work correctly with old hard drives.

What you are running into is the usual retro IDE hardware being a pain in the ass. The farther back in time you go, the more trouble you are going to have. IDE CDROMs were very rare or non-existent when this card came out. ATAPI was probably a fresh new idea. Your problem is probably ATAPI issues or lack of support for it.

You'll be better off getting a I/O card that has a secondary channel. Usually the secondary channel is meant for devices other than HDDs. On the EIDE2300+, for ex, the primary channel is VLB-based and has lots of tweaks for HDD speed (cool back then). The secondary channel is a more proven ISA design. None of this old hardware ever works quite as well as it sounds like it should though so good luck.

Very interesting. 😀 I'm looking at a quad speed IDE CD-ROM drive here, and there's a manufacturing date of October 1995 written on it. I think quad speeds were the first batch of IDE CD-ROM drives, the ones before that were all those (1/2x speed) different propriatory brands like Sony, Mitsumi, Panasonic, Creative, maybe more? The VLB controller card in question is circa '94, I think.

So, picture someone like me back in time in late '95. I've just come back from the shops with a brand new quad speed IDE CD-ROM drive, and what do I plug it in to? My Soundblaster's no good, even a later CT22xx model, because it doesn't have an IDE or CD-ROM drive connector, just some of those propriatory ones mentioned above. My VLB IO controller's no good, because of the problem outlined in the O.P. So...how do I use it? I guess the answer is (as explained by @swaaye) to upgrade the VLB IO controller card, to get one with a secondary channel. (Is this one reason why some people "went SCSI"? 😉)

Or...you could waste an ISA slot by getting a dedicated CD-ROM controller card.

Or...would this also work: swap out the VLB IO controller for an ISA-based multi IO controller? (I'm sure I've had this master slave HDD/CD combo working with one of those old things.)

You'd usually plug it into a sound card with IDE (popular in '95) or your secondary IDE port on your computer. Onboard IDE in 1995 was getting a lot better in Pentium systems, with Intel chipsets especially. Triton 1 was around (430FX) with PIIX IDE.

I believe I ran a 4X CDROM on my secondary IDE channel on my Promise EIDE2300. A 4X CDROM only pulls at most 600KB/s which is low enough not to be bottlenecked by ISA. I remember my drive too, it was a troublesome Teac CD55A with terrible access times and lots of CPU load, but it was cheap.

You just need to find a VLB IO card with dual IDE channels.