This usually means an average hard disk access time of 19 milliseconds or better. Thermal Calibration Most hard disks automatically periodically perform a thermal calibration to ensure proper functioning. Calibration interrupts hard disk operations for as much as one and a half seconds. Some hard disks force a calibration at fixed intervals, even if the disk is in use, causing interruptions fatal to real-time operations such as CD writing. This problem arises particularly when the amount of data to be written is large.
Some hard disks calibrate intelligently, postponing it when the disk is being used.
Fujitsu and many Micropolis hard disks work this way. Data is transferred from the computer's hard disk to the buffer memory of the CD recorder, and thence to the recording laser, which inscribes pits into the surface of the CD at a steady speed which cannot be altered or interrupted. When you write a real ISO image from hard disk to CD, speed is rarely a problem because the image is already one gigantic file in which the files and structures are already in order and divided into CD-ROM sectors, so it is only necessary to stream off the data to the recorder. When you write a virtual image, things get trickier. Now, in order to write to CD, the program must consult its database to find out where each file should go in the image and where it actually is stored on hard disk.