Justa Picker is wrong about what buffer underrun is.

Buffer-Underrun

When writing a CD, the data has to be written continuously, i.e. there may be no breaks in the data flow. At e.g. 2x burn speed this means that the data is written at 352 KB / second. As of course the computer can not supply a constant stream of data (as it transfers it block by block), the CD recorders have got a buffer which temporarily stores the data sent by the computer and then sends it on to the writer at the correct speed.

If the computer does not manage to keep refilling the buffer during the write process (i.e. the buffer is emptied completely), then the write laser does not have any more data to write so the write process has to be interrupted. This situation is called a "Buffer Underrun". Unfortunately it is not usually possible to "restart" the write laser (unless the recorder is "Burn-Proof"-capable), which means the CD can not be written on any more after a Buffer-Underrun, it is defective. (the parts which have already been recorded can usually be played)