Because it has some burn errors.
Keep the plates spinning
One of the few consistent virtues of computers is patience. They may rudely insist that we answer “OK” or “Cancel” to some inane question we don’t understand, but they don’t complain if we wait hours before answering.
Burning a disc is a rare exception. It is a physical process, with the disc being spinning at up to 4,800 revolutions per minute1. Then the drive must write retrieve the information to burn just as it needs it – up to 900 megabytes worth. There are two ways that this can go wrong: (1) physically shaking the disc as it spins, or (2) interrupting the data transfer.
1. Physical trauma
Modern drives do their best to lock the spinning disc in place, but this is not an excuse to torment it. Do not pick up your laptop and carry it around while it is burning a disc. Do not pick up your desktop either, but you’re less likely to do that by accident.
Even stamping on the floor while your computer is burning a disc can shake it around so much that the burn fails.
2. Attention deficit
The speed of a CD drive is its multiple of audio playback speed. If a 900 megabyte CD-RW can hold 99 minutes of audio, a 24× burner can fill up that disc in 4 minutes and 8 seconds. The computer must provide the information to be written at 220 megabytes per minute, or 3.6 megabytes per second.
This is usually no problem. The data to be written are stored on your hard drive2, which can hopefully deliver a sustained data transfer rate of 125 megabytes per second3.
But that is a sustained transfer rate. If we launch a new application or open a large document, the hard drive must seek to dozens or even hundreds of locations and read data into memory before it can retrieve more CD contents. Even if we don’t tell the computer to do anything, regular maintenance (especially anti-malware scans) can have the same effect.
The burn process will maintain a large “buffer” of information, which usually lasts until it can ask the hard drive for more. However, if the computer is having a bad day, the interruption may end in “thrashing”, where it the hard drive must repeatedly seek to different locations. You can usually hear this as a continues rattling sound.
But the disc passed verification and reads fine on my computer!
Drives are good at reading the discs they created. The verification step is good, but it cannot detect quirks.
What can I do?
- Do not shake up your computer or the room it sits in while burning.
- Do not use your computer for other tasks while burning.
- Reduce your burn speed.
The first two are simple enough – just stop doing things. What about the third?
In iTunes, you can choose your “Preferred Speed” just before burning:
In Windows Media Player you must open the Options and change the burn speed before starting your burn:
If you find that this helps, experiment to find the fastest burn speed that you can safely use. Remember, slowing right down to 1× will mean the burn will have the same duration as the music you are burning – 99 minutes if you fill up the disc.
Only take the steps above if you are having trouble burning discs. Otherwise you may waste hours hiding from your computer while it burns discs at 1×, to no useful purpose.
If these steps don’t help, your CD-RW drive may need repairs.
- Wikipedia entry on CD-ROM Transfer rates [↩]
- If you have multiple gigabytes of RAM, it is also remotely possible that your CD contents may be entirely cached in RAM [↩]
- Wikipedia entry on Hard disk drive Data transfer rate [↩]