I can fully confirm Rebel2k's buffer underrun problems: When I use the HP dual layer firmware, the buffer underrun protection simply does NOT work. The light of the drive stops flashing when the buffer is empty. So far, so good, but when the buffer is full again, the drive does NOT continue to burn :Z . This error occurs with HP's dual layer firmware 2.21 only and Herrie's releases which are based on this firmware.
Firmware 1.27 for the HP 420n and all other NEC firmwares work perfectly. I know this sounds pretty weird, but there is absolutely no doubt that this issue is related to the firmware.
Obviously, it has nothing to do with Rebel2k's USB enclosure since my drive is internal.