The P and Q channels are used to tell an audio player how to play back an audio disc. The Q channel contains the index markers. When a track begins, the index marker changes to 1. The TOC does NOT show the length of each track, only its starting point. As a hard disk works, it heats up.
Most materials expand when heated.
This means that a hard disk's read and write heads must be periodically recalibrated. The thermal calibration operation calibrates the hard drive's servo system to ensure that the disk's read and write heads remain precisely over the data tracks, compensating for temperature changes during normal operations. The thermal calibration operation, depending upon the hard disk, may require many hundreds of milliseconds to complete. During calibration, no data is read from or written to the hard disk, so during all or part of this process the drive will not be responsive to data requests. If the thermal calibration process takes too long during CD recording, the CD recorder's buffer will be emptied, and a buffer underrun or other recording error may occur.