When I installed the demo of ACDSee Pro, sometime in 2008, I believe it installed PFC.SYS as a lower filter for CD/DVD drive access, and it did not uninstall it when I uninstalled the program. This screwed up access to DL disks on a DL drive until I got rid of the registry entry.

It's a while ago, so I am willing to be corrected on this ... but I wanted to note this just in case someone else might run into the same problem.