DVD RW Drive no longer recognizes DVD disks

I had been successfully reading and burning DVDs with two separate external
USB DVD RW drives until something happened yesterday that suddenly prevented
my DVD drives from recognizing DVD disks (They both still recognize CDs
okay.) If I insert any DVD (whether blank or a commercial movie, etc.) into
either DVD drive, I now get an error message saying, "Please insert a disk
into drive [L: or J: depending on the drive]." I can't think what I may have
done in the last 24 hours to cause this change. I've uninstalled and
re-installed the drives, and I've tried updating drivers (no updates were
necessary), to no avail. That both drives are similarly affected suggests to
me that the problem lies not with either drive (installation, drivers,
connections, etc.) but rather with the system. Any ideas?

--
jim2148 [I am using a stand-alone desktop powered by a Pentium 4
(1.8GHz), Win XP Home SP2]