I've put together a spare computer from spare parts. The CD drives work fine when used as the boot disk (live cd or install cd), but will not work when I boot from the hard drive.

I've tried Mythbuntu, Ubuntu, Debian stable and Debian testing, and they all can access the CD drive when booted off of it as an installer or Live CD. But, if I install them to the hard drive and boot from there, they can't access the CD drive.

I currently have Debian testing installed. The exact error message I get is:
(as regular user) $> mount /media/cdrom0
mount: special device /dev/cdrom does not exist

After searching for similar problems online, I tried referring to different devices in the /dev directory:
(as root) #> mount /dev/sdc0 /media/cdrom
mount: no medium found on /dev/sr0

I tried swapping out the CD drive with another, and I've tried a variety of pressed (as in, factory-burned) data CDs, but the results are always the same.

The motherboard is a nforce 570 slit-a ver 5.01. The CD drives are PATA, and the hard drive is SATA.

I realize it may be bad hardware, but I was hoping it was a software/config issue, like a finicky chipset on the motherboard that needed a bit of a workaround. Has anyone seen behavior like this before?

PS: My best guess is that the PATA controller on the motherboard is acting up, and there's something different about how the CD drive is accessed when used as the boot device (where the motherboard has to make it accessible) vs when the computer boots from something else and the it's up to the OS to load ATAPI/whatever drivers. I don't know if that's how booting & cd drives work; just grasping at straws here.

Most modern distros I know of refer to the first CD/DVD-drive as /dev/sr0, may be you should try that.

In first command you get information that /dev/cdrom file is not exist, in my system it is symlink to /dev/sr0. And second there are no medium inside. Did you tried mounting it with cd inside drive?
Insert some data CD into drive and try as root: Make sure /media/cdrom directory was created. To get information which device is your cdrom look at dmesg output.

Ok, I tried mounting using /dev/sr0 instead of /dev/scd0 as eSelix suggested, but I get the same message.

Yeah, I've always got a CD in the drive. Good catch, though. The simplest things sometimes...

For what it's worth, there is only one CD-ROM in the system. When I switch them, I disconnect and physically remove the one I don't want in there.

I've also discovered that "ls -ld /dev/scd0" shows that it is a symlink to sr0. There's no "/dev/" before that "sr0", but I'm assuming that's OK. Also, "ls -ld /media/cdrom" shows it is a symlink to cdrom0 (again, with no path {"/media/"} in front of it).

I ran "dmesg | grep -i cd" and found the following lines referring to the CD-ROM. Note that I don't have a web browser installed on the machine, so I'm typing these into another PC where I can see the problem PC's screen. I didn't bother typing in the timestamps from dmesg:

"scsi 4:0:0:0 CD-ROM HL-DT-ST DVDRRW GSA-4166B 1.00 PQ: 0 ansi: 5"
"sr0: scsi3-mmc drive: 48x/48x writer dvd-ram cd/rw xa/form2 cdda tray"
"cdrom: Uniform CD-ROM driver Revision: 3.20"
"sr 4:0:0:0: Attached scsi CD-ROM sr0"

Will it find a cdr? If it finds a cdr on boot and doesn't find a pressed disc on data, you've left out something essential from the kernel.