During the write process, the entire disc is not written at one time. The disc is spun and a moving laser procedurally writes to the disc’s “track” bit by bit. For this reason, the data to be written is only sent to the recording device at a rate it can handle. The data going to the laser to be physically written is coming from a “buffer”, or specialized storage memory, which acts as “batch” of all the data to be written from the premaster. The computer’s CPU needs to constantly be refilling this buffer with the next “batch” as the previous is written. However, if another high-priority process on the computer keeps the CPU from refilling the buffer in time, it can empty.
An empty buffer leaves the laser with nothing left to write, but the record process hasn’t stopped. This results in a “buffer underrun” error and a failed write. Sometimes, the data written up to that point is still readable. Other times, the disc is a complete “coaster”. The user needs to start over with a fresh disc.
To prevent a buffer underrun, it was generally advisable to not run any other intensive processes while writing a disc, burn at a slower speed, leave the machine alone when writing and even defragment the hard disk (as disk delays can prevent the buffer from being filled in time.) The greatest cause of buffer underrun events, however, is a busy or slow CPU.
Not long after recording-capable CD drives came onto the market, technologies attempted to address this issue. They were given a few different PR names, including “Burn Proof”. These aim to prevent buffer underrun events during the writing process by stopping the write process if the buffer is about to become empty. During this time, the buffer is untouched and allowed to refill with the next portions of the premastered data and/or audio. Once the buffer is full again, the writing can continue. Some implementations used specialized memory on the drive itself acting as a hardware-level buffer and adding an extra level of protection against buffer underrun events.
Though previously a very common event, buffer underrun events are no longer of much concern. Modern computers usually contain at least two processor cores and at least two logical threads within their CPU. These allow the system to perform other tasks while writing a disc without risk of leaving the write buffer to empty. Therefore, multitasking on the machine during a disc burn operation is less likely to cause a buffer underrun event. Other writing errors do still exist, however.
Dirt or scratches on the media when writing can leave errors. Dust on the laser could also have this effect. A bad premaster may exceed the capacity of the disc, resulting in a bad, often unusable, disc. Some drives, particularly very old ones, will not read discs that have not been “closed” after the write process. Other drives cannot read CD-RW discs (or only certain brands.) This can sometimes be corrected with a manual adjustment within the drive.
Thankfully, media writing nowadays is much simpler and more reliable than it used to be. It remains to be seen, however, how long optical media technology is going to stick around!