Thank you very much!
Ok, in order:
Both optical drives are UDMA Mode 4. I have no other PATA drives in this system.
The drive (both, for that matter) is clean and functioning normally everywhere in the OS but DVDFab. The Pioneer drive will play a DVD-Video just fine, such as in WMP. I'm running Vista 32b, by the way.
I always start DVDFab first, then insert discs; this has been failing for a while now for the Pioneer drive, but the other drive (the Plextor DVD-RW) still works with this method. I attempted to insert a DVD first before loading DVDFab, but that also failed.
Just to point this out, when I have DVDFab running and insert a DVD, it opens the reading progress bar for a split second, then stops attempting to read the disc; in the "Info" screen it always displays the disc's correct title and "Disc is not opened."
Disabling Autoplay did not change the situation at all, i.e. DVDFab still tries to open the disc for a split second and fails.
I do not really have any other optical-disc-related software; I have DVD Shrink, CD Burner XP, and Exact Audio Copy. I did not install any new software that I know of between the time that DVDFab was last able to read from the Pioneer and the time that I noticed it was no longer able to do so. I will again mention that I did update DVDFab and immediately thereafter the problem was noticed, but it had possibly been a while since I had last used DVDFab and could not say that the new version was immediately culpable. Also, returning to a previous version of DVDFab did not solve the problem.
What exactly am I looking for in terms of the device filters? I checked what I assume is the correct registry key and there are no upper or lower filters even listed.
To answer a PM, the problem is with one optical drive only (the Pioneer DVD-ROM; the Plextor DVD-RW works fine in DVDFab) and with ALL DVDs, even previously-created DVD-Rs. In other words, DVDFab will read any DVD-Video in the DVD-RW only, and when I create a DVD-R it will be able to read it with the DVD-RW but not the DVD-ROM.
Again, thanks in advance to all!