hard disk - Computer Definition
The primary computer storage medium, which is made of one or more aluminum or glass platters, coated with a ferromagnetic material. Although the terms "hard disk" and "hard drive" are used synonymously; technically, the disk spins inside the drive. All computers have an internal hard disk for storage, and hard disks in external cases can be plugged into a USB, FireWire or eSATA port for additional storage. Slowly but surely however, hard disks are being replaced by non-mechanical drives (see solid state drive). Today's hard disks are "fixed," which means their platters reside permanently in the drive. In the past, removable cartridges were used for backup and transport (see removable disk). Storage... Not Memory Hard disks are not the computer's main memory. Disks store programs and data until deliberately deleted by the user, but memory is a temporary workspace. To learn how this workspace is used to process data, see memory. For a summary of memory and storage types, see storage vs. memory. Capacity and Speed Hard disks rotate constantly from 4,000 to 15,000 RPM; however, to preserve battery or power, they can be configured by the user to turn off after a defined period of inactivity. Capacity is measured in bytes, and the largest drives have passed the terabyte threshold. Speed is measured by how long it takes to begin transferring data; approximately three to 15 milliseconds (by comparison, CDs/DVDs take 80 to 120 ms) and the rate of transfer is measured in hundreds of megabytes per second. See hard drive capacity, access time and transfer rate. Hard disks are pre-formatted at the factory, which divides the platters into identifiable sectors. For more details on disk structure, see magnetic disk, format program, hard disk defect management and drop protection. Hard Disk Types Over the years, several kinds of hard disks have been employed. Today, SATA drives are the most common, although SAS drives are also used. For more details, see SATA, SAS, SCSI and hard disk interfaces.