To exclude problems with the drive itself, try it out as an IDE device on a PC instead of your laptop. You can perhaps persuade a friend to let you do this if you don't have a regular PC. If you get higher speed burns, then the drive should be fine. If the drive is not at fault, you could always try out another enclosure to remove the enclosure from the list of suspects.
My suspicions point to the external enclosure. There have been many reported problems with budget enclosures producing coasters and reduced performance. One of the Nero engineers is in an ongoing discussion with users who reported problems but the issues still haven't been resolved as far as I know. Issues identified but not producing a complete fix are:
- drive enclosure power supplies not producing enough power for high-speed writing
- third-party ASPI drivers conflicting with Nero ASPI driver
- various chipset incompatibilities with external enclosure devices
- people using older versions of Nero
- Windows XP not having latest USB updates applied
- users not updating device firmware
It's by no means an exhaustive list and the problems have not yet been ironed out. But I hope this can help you resolve matters even if only to confirm the drive is ok.