Ever since the creation of Linux in 1991, the way to mount IDE hard disks and CD-ROMS was to type in something like mount /dev/hda1 /mnt, or for an IDE CD-ROM drive, something like mount /dev/hdc /mnt. However, this no longer works that well in the kernel that comes with Ubuntu 7.04 (2.6.20-16-generic). If there is a DVD which is larger than four gigabytes in size, the CD/DVD-ROM drive can not access the files past the four gig limit. This was not a problem with older Linux kernels; the kernel developers broke something that has been working for years.
Instead, one has to get the udftools package via sudo apt-get install udftools, configure the file /etc/default/udftools to point to the CD/DVD-ROM device, and then mount the DVD as /dev/pktcdvd/0. Ugh. As an aside, there are online guides for making UDF packet-writing DVDs (DVD-RWs that you can write to as if they were a huge floppy disk) that give bad directions. Please read the directions in the file /usr/share/doc/udftools/README.Debian.gz for correctly formatting a packet DVD. I now have two coasters that used to be DVD-RW blanks because I did not follow these directions, but instead trusted inaccurate on-line directions. In particular, I didn't do the essential growisofs -Z /dev/hdc=/dev/zero step between formatting the DVD-RW and putting the UDF filesystem on the DVD-RW, since the on-line guide did not mention this step.
Also, when mounting a DVD-RW, you really want to mount it like mount -o noatime /dev/pktcdvd/0 /mnt since otherwise it will do very slow writes when all you want to do is read some files. Yes, I actually learned something useful from reading a Slashdot discussion. Naturally, Slashdot misattributed a quote to Linus, but hey at least the information in this thread is useful.