Blank media is embedded with a "media ID" that the recorder reads to see exactly what the disc is (who made it, how fast), which it then uses to determine exactly what speed it can record on it. However, if the firmware doesn't know about a particular media or speed, then it often simply won't work. From what I hear the the VX500 will not write to 16X media and must use 8X or slower.

All it needs is a firmware update to recognize 16X media, but everything I've read says that isn't going to happen.