I never got NT 3.1 to see any ATAPI CD-ROM drives.
Back then, SCSI CD-ROM drives and proprietary interface were much more common.
If my memory serves me well, the Trantor SCSI interface of my PAS16 and the controller card of the Mitsumi LU005 were supported, among others..

Another strange thing is, I've managed to find the data for the SP6 floppy ATAPI.EXE (From https://www.helpjet.net/Fs-85106202-75843699- … 82-extract.html ) that's supposed to improve large hdd support. I've opened it with 7-zip, extracted it's contents, then copied that to a freshly generated 1.44MB floppy disk image by UniPCemu.

But when selecting Other instead of Standard PC from disk 1(pressing F5 while detecting hardware and after it's starting to load as well(just to be sure), inserting the disk into the A-drive, it tells me it can't find any supplements(or something like that) on the disk? It then tells me to insert disk 1 again and goes back to the selection menu?

That happens both on NT3.1 and NT4.0?

Just found out some bug in my ATAPI emulation. The result of the Read Capacity command was backwards, in little endian format, instead of big endian format(as is supposed to be the case). Whoops!

Edit: The same appears to have been wrong with the sector size reported by the very same command.

Although oddly enough, when loading itself, AMI Diagnostics 5 seems to insist on reading past the end of the disk with the "Initializing CD Tray Test ..." at the bottom(the loading stage)?

I do see some EPM(End of Media) bit in the error register that's not used. How does that bit work with ATAPI?