Cause: Almost all CD/DVD recorders come with a small amount of buffer memory to smooth out the burning process. The buffer reads ahead and provides the data required for burning in a lin ear manner. A buffer underrun error happens when the data source slows down for some reason. Since the CD is spinning and there is no data to be burnt, it causes the burning process to be aborted, resulting in a coaster.
Solution: Unless you have a really old drive, you shouldn’t see this error. In fact, almost all recorders today have hardware support to ensure buffer underruns don’t occur. Moreover, burning software also have support for this feature - make sure you activate it; it’s just a matter of checking a box. If you have a drive that does not support this feature, make sure you use your hard drive as the data source, since a CD to CD copy will definitely result in a coaster. Also, defragment your hard disk regularly to avoid a slow-down during the burning process.