All NEC DVD burners store an internal drive identification in their Flash RAM. In order to cut license costs, some drives were sold without the ability to write to DVD-R, DVD-RW or DVD-R DL discs. Althought by means of hardware these drives are absolutely identical with there +R counterparts, it was impossible to burn these discs even when applying the "full version" firmware.
Some modified firmwares exist that allow crossflashed drives to burn the unsupported discs, but each time you want to flash a newer firmware again, this one needs to be patched too. It is possible however to modify the internal drive identification and trick the drive into accepting minus discs permanently. This feature was at least reported to work with NEC ND3100A drives.
If you want to upgrade (or maybe even downgrade) your drive, just download Liggy's NEC drive converter. Please note that changing this drive ID with the Drive Converter is only half the job. You still need to flash an appropriate firmware for the drive that corresponds to your new firmware ID.
Please note that it is IMPOSSIBLE to enable DVD-RAM support on 3550 and 3551 drives or Labelflash on 3550 or 4550 drives by using this drive converter.