Scans - Mitsubishi 6x DVD-RW

Asus DRW-1608P @ 6x
Asus DRW-1608P2S @ 6x
Medium write error during test - no result
NEC ND-3500A @ 4x
NEC ND-3540A @ 6x
NEC ND-4550A @ 6x
Plextor PX-716A @ 4x
PX-712A: cannot scan disk - disk is not recognised
Plextor PX-755A @ 6x