The mechanism that reads and writes data on a hard disk. Hard disk drives (HDDs) for PC's generally have seek times of about 12 milliseconds or less. Many disk drives improve their performance through a technique called caching.

Ultra ATA/66, the newest version of the ATA standard, supports burst mode data transfer rates of 66MBps and is standard. Disk caching can dramatically improve the performance of applications, because accessing a byte of data in RAM can be thousands of times faster than accessing a byte on a hard disk. When a program needs to access data from the disk, it first checks the disk cache to see if the data is there.