(HDD) is a non-volatile, random access device for digital data.

1. HDDs record data by magnetizing ferromagnetic material directionally. Sequential changes in the direction of magnetization represent patterns of binary data bits.
2. The platters are spun at speeds varying from 3,000 RPM in energy-efficient portable devices, to 15,000 RPM for high performance servers. Information is written to, and read from a platter as it rotates past devices called read-and-write heads that operate very close
3. The disk drives use a sealed head/disk assembly (HDA) which was first introduced by IBM's "Winchester" disk system. The use of a sealed assembly allowed the use of positive air pressure to drive out particles from the surface of the disk, which improves reliability.
4. A typical hard disk drive has two electric motors; a disk motor to spin the disks and an actuator (motor) to position the read/write head assembly across the spinning disks
5. The silver-colored structure at the upper left of the first image is the top plate of the actuator, a permanent-magnet and moving coil motor that swings the heads to the desired position
6. If the mass storage controller provides for expandability, a PC may also be upgraded by the addition of extra hard disk or optical disc drives.
7. Typical hard drives attempt to "remap" the data in a physical sector that is going bad to a spare physical sector—hopefully while the errors in that bad sector are still few enough that the ECC can recover the data without loss.