Recordable CD and DVD media are made of several layers of different materials. In addition to the label surface, a disc is comprised of a protective coating, a layer of reflective material, a layer of dye/polymer material, and a thicker transparent substrate. The dye/polymer layer changes when a specific kind of laser is focused through the transparent layer and onto it.
To write data onto a disc, the optical drive uses this type of laser to make a series of microscopic marks in the dye. The resulting sequence of light and dark spots (called "pits" and "lands") represent the digital ones and zeros that comprise your data. To read a disc, the optical drive shines a different light on the disc surface, and the sequence of pits and lands are reflected back and read as information.
Recordable and rewritable CDs and DVDs are manufactured with a small amount of written "pregroove" data that contains various disc attributes, including the disc type, part number, manufacturer, the kind of dye used, its writable capacity, rated write speed, and other items. This data resides in a very small area on the disc that is not part of the user-writable area. The optical drive reads this data to determine how it should handle the disc.
When you begin the burning process, the drive will burn some very short tests in another tiny area of the disc, called the power calibration area. It does this to estimate the optimal setting for its laser so that the spots burned into the disc's dye will yield ideal reflectivity when reading.
Factors that affect disc burning performance
Despite the very precise specifications for manufacturing recordable discs and the drives that handle them, it is possible for issues to occur. Here are some things that can impact your disc burning.
Due to slight mechanical differences between different optical drive models, the same disc might not perform exactly the same way across different computers that have similar drives. One way that two similar drives can differ is the firmware. If a firmware update is available for your optical drive, apply the update. Firmware can help define the way an optical drive assesses and reacts to discs.
The software that you use to burn a disc has its own set of rules about how it interprets and responds to data it receives from the drive about the disc. Based on these rules, the software can tell the drive to burn the disc, burn at a reduced speed, or not allow the burn to occur.
Some software may request that the drive perform a low-speed burn if conditions do not allow it to burn at the rated speed (of the disc or drive). The software can request a certain burn speed, or it may allow the user to choose a speed for the drive to attempt. It's possible for software to send a request for the drive to burn at a speed that is below or above the rated burn speed of the disc. Advances in optical drive technology have enabled newer models to offer faster burn speeds. Generally, older optical drives are not able to burn at the faster speeds featured in more recent models.
Optical disc drives use motors to spin up discs to their maximum revolutions per minute (RPM). As the disc spins, the laser starts writing from the inner portion of the disc to the outside. As the burn reaches the outer portion of the disc, the drive is able to write at faster speeds, as more pits and lands are available in one revolution. Maximum burn speeds are reached toward the outer portions of the disc. Therefore, if you burn a disc with a small amount of data, it may not reach the maximum burn speed because the written area doesn't approach the outer edge of the disc.
There are a number of other factors that can affect the drive's ability to perform an accurate burn at the fastest possible speed. These include, but are not limited to:
- Rated burn speed of the disc
- Requested burn speed from the software
- Quality and consistency of the disc's dye (even with the same brand of disc, batches may vary in quality from one to the next)
- Quality and consistency of the disc's coating
- Width and consistency of the disc's thickness
- Exact placement of the center hole (the smallest variations from "perfect center," or variation in disc thickness can cause wobble)
- Age of the drive
- Temperature of the disc and temperature of the drive
- Dust and humidity in the operating environment
Note: Apple disc media is manufactured to exacting specifications, and may be generally suggested as a test for quality.
Tips for using your optical drive
- Make sure you've installed the latest firmware updates, and that you are running the latest version of the Mac OS and your disc-burning software. Apple updates are available here.
- If you're having issues, try the following troubleshooting steps:
- Open the Utilities folder inside your Applications folder, and open the application called Console. You can look for more information about any error that occurs when you burn discs in the console.log file.
- Make sure that the disc you are burning is clean (wipe it with a soft, dry, lint-free cloth).
- Do not move, bump, or shake the computer while a disc is being burned.
- Use blank discs that are rated for the burn speed of your drive, or consider trying a different brand of disc.
- Specify a burn speed that is lower than the maximum speed rating for your optical drive.
- Quit all open applications that you are not using.
- Restart your computer and try burning the disc again.