Recently, Plextor has introduced "protected commands" into these latest drives. Commands are enabling/disabling Bitsetting, GigaREC or SilentMode, as well as all quality check commands.
Plexluthor patched the firmware for the PX-755A and PX-760A to disable this protection. I have added them in the database. More infos here http://www-user.tu-chemnitz.de/~noe/Plextor/index.php
Added firmware for :
Plextor PX-760A - 1.01
Sony DW-G120A - MYL2