It seems a little random.
Mostly, if I've burned a CDRW in another burner, or it's been lying around for a while, the Plextor can't erase it.
If it's a CDRW/DVDRW I've done in the Plex, then it usually erases okay.
The Plex has really bad handling of RW media at the moment - Had one that would lock up the drive when inserted, but a Quick Erase in my old LiteOn and it worked in the Plex again...
I hope they fix this...!