A tape drive is a data storage device that reads and writes data on a magnetic tape. Magnetic-tape data storage is typically used for offline, archival data storage. Tape media generally has a favorable unit cost and a long archival stability.

A tape drive provides sequential access storage, unlike a hard disk drive, which provides direct access storage. A disk drive can move to any position on the disk in a few milliseconds, but a tape drive must physically wind tape between reels to read any one particular piece of data. As a result, tape drives have very large average access times. However, tape drives can stream data very quickly off a tape when the required position has been reached. For example, as of 2017^[update] Linear Tape-Open (LTO) supports continuous data transfer rates of up to 360 MB/s, a rate comparable to hard disk drives.

Magnetic-tape drives with capacities of less than one megabyte were first used for data storage on mainframe computers in the 1950s. As of 2018^[update], capacities of 20 terabytes or higher of uncompressed data per cartridge were available.

In early computer systems, magnetic tape served as the main storage medium because although the drives were expensive, the tapes were inexpensive. Some computer systems ran the operating system on tape drives such as DECtape. DECtape had fixed-size indexed blocks that could be rewritten without disturbing other blocks, so DECtape could be used like a slow disk drive.

Data tape drives may use advanced data integrity techniques such as multilevel forward error correction, shingling, and linear serpentine layout for writing data to tape.

Tape drives can be connected to a computer with SCSI, Fibre Channel, SATA, USB, FireWire, FICON, or other interfaces.^{[lower-alpha 1]} Tape drives are used with autoloaders and tape libraries which automatically load, unload, and store multiple tapes, increasing the volume of data which can be stored without manual intervention.

In the early days of home computing, floppy and hard disk drives were very expensive. Many computers had an interface to store data via an audio tape recorder, typically on Compact Cassettes. Simple dedicated tape drives, such as the professional DECtape and the home ZX Microdrive and Rotronics Wafadrive, were also designed for inexpensive data storage. However, the drop in disk drive prices made such alternatives obsolete.

Magnetic tape is commonly housed in a casing known as a cassette or cartridge—for example, the 4-track cartridge and the Compact Cassette. The cassette contains magnetic tape to provide different audio content using the same player. The outer shell, made of plastic, sometimes with metal plates and parts, permits ease of handling of the fragile tape, making it far more convenient and robust than having spools of exposed tape. Simple analog cassette audio tape recorders were commonly used for data storage and distribution on home computers at a time when floppy disk drives were very expensive. The Commodore Datasette was a dedicated data version using the same media.

Manufacturers often specify the capacity of tapes using data compression techniques; compressibility varies for different data (commonly 2:1 to 8:1), and the specified capacity may not be attained for some types of real data. As of 2014^[update], tape drives capable of higher capacity were still being developed.

In 2011, Fujifilm and IBM announced that they had been able to record 29.5 billion bits per square inch with magnetic-tape media developed using Barium Ferrite (BaFe) particles and nanotechnologies, allowing drives with true (uncompressed) tape capacity of 35 TB.^[17][18] The technology was not expected to be commercially available for at least a decade.

In 2014, Sony and IBM announced that they had been able to record 148 billion bits per square inch with magnetic tape media developed using a new vacuum thin-film forming technology able to form extremely fine crystal particles, allowing true tape capacity of 185 TB.^[19][20]

On December 15, 2020, Fujifilm and IBM announced a Strontium Ferrite (SrFe) technology able, in theory, to store 580 TB per tape cartridge.^[21]