A CDE Definition
15K RPM disk
A high-speed SATA or SAS hard disk drive that spins at 15,000 revolutions per minute. See hard disk.
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 used to have an internal hard disk for storage; however, today, storage can be solid state (see solid state drive). External hard disks can be plugged into USB or a eSATA port for more storage.
Hard disks are "fixed disks," which means the platters reside permanently in the drive. In the past, removable cartridges for backup and transport contained the platters (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 (RAM) 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
Capacity is measured in bytes, and the largest drives hold up to 10 terabytes. Speed is measured by transfer rate in megabytes per second as well as latency: how long it takes to begin transferring data, typically 3 to 15 milliseconds (ms). By comparison, CDs/DVDs take 80 to 120 ms.
The platters rotate constantly at thousands of RPM; however, to preserve battery, laptops default to powering them down after a set period of no activity. This mechanical action is why hard disks are slowly fading into history and being replaced with SSDs (see solid state drive). See hard drive capacity, access time and transfer rate.
Disks Come Pre-Formatted
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 emerged. Today, SATA drives are the most common, although SAS drives are also used. For more details, see SATA, SAS, SCSI and hard disk interfaces.
Non-Removable Internal Hard Disk
First Hard Disk Computer (1956)
Five Megabytes Total
First Personal Computer Hard Disk (1979)
Four Decades After RAMAC (1998)
World's Smallest (2005)
Before/After Your Search Term
|13W3 connector||16/44.1 kHz|
|15 ATM||16-bit color|
|15-pin D-sub||16-bit computer|
|1550 nm||16-bit computing|
Terms By Topic
Click any of the following categories for a list of fundamental terms.