Dell GX60 reads CDs but not DVDs

I have a "small desktop" Dell Optiplex GX60 here which I purchased from a school as a retired unit. It was manufactured around 2003/2004 and it is on BIOS revision A02. It was not provided with an optical drive, so I installed a DVD-ROM using the provided ribbon and power source. This DVD-ROM was previously in my desktop PC and worked perfectly. The problem I am having is that the DVD-ROM drive will read/boot off of CDs but not DVDs. As I said, the drive reads DVDs fine, but the computer will not acknowledge their existence.
I then swapped to another DVD-ROM drive, again one that we in my desktop and is working perfectly, and this one also produced the same results, ruling out the DVD drive, or the cable.
In both cases, the drives were recognised by the BIOS as CD-Readers, and as I assume is the requirement, both drives were installed, one at a time, in cable select mode with 1 device per IDE cable.
Currently the device is acting as a smoothwall firewall, but the problem is either hardware or BIOS I/O based as the PC will not boot from DVDs but it will boot from CDs.

 

Thanks for your reply. The problem here is that I do not have a floppy drive to create floppy images, or floppy disks to put it on, so my only option to update the BIOS is to change the hard drive for a blank one, install XP and run the hard drive executable, then put my smoothwall hard drive back in. I would be more willing to do this if I knew the A09 BIOS resolved the issue I am having but apparently Dell is not willing to part with that kind of information.