I'm trying to copy about 500 pdfs from a DVD into a folder on my desktop. I have no problem doing this until after about 5 minutes, in which the disc will unmount by itself, thus screwing up the transfer and giving me a basic I/O error. It doesn't eject the disc, though. I have tried to copy the files over 4 times now and the same thing happens after about 5 minutes. I'm running Gnome 2.22.3 on Debian Lenny if that makes a difference. What could the problem be?

Are you sure the disc is not corrupted in any way? Do the system logs show anything around the time the disc unmounts?

Is the disc getting mounted through HAL/UDEV (or in other words, does it mount automatically) or are you manually mounting it?

I have no idea if it's corrupted but I don't see how it would be. I checked syslog and I see:

Sep 1 13:17:44 OSIRIS kernel: ATAPI device hda:
Sep 1 13:17:44 OSIRIS kernel: Error: Not ready -- (Sense key=0x02)
Sep 1 13:17:44 OSIRIS kernel: No reference position found (media may be upside down) -- (asc=0x06, ascq=0x00)
Sep 1 13:17:44 OSIRIS kernel: The failed "Read 10" packet command was:
Sep 1 13:17:44 OSIRIS kernel: "28 00 00 00 00 14 00 00 01 00 00 00 00 00 00 00 "
Sep 1 13:17:44 OSIRIS kernel: end_request: I/O error, dev hda, sector 80

numerous times. I've never copied a lot of files off of a disc before so it may just be this disc. As for your last question, I had it mount automatically the first few times and then did it manually; both with the same result.

Well those error messages can mean only two things really. Either the disc is corrupted, or your optical drive is not functioning correctly.

If you can read files from other discs fine, I would say more than likely the disc is corrupted somehow. This may seem like a ridiculous question, but I assume you have checked to make sure the disc is clean and there are no scratches? What about the drive, have you tried cleaning the laser lens?