Originally Posted by NooNoo
If you are copying on the fly, then the communication between the two drives is not fast enough causing the buffer to empty faster than the source drive can fill it.
If you are copying from the hard drive, the same problem exists - the hard drive cannot supply data to the dvd drive via the buffer fast enough.
Most dvd burning packages have buffer underrun protection capabilities, but it is linked to the firmware on the drive itself.
What burning software are you using? What make/model dvd drive and what is the make/model of the source drive?