A magnetic tape drive enables storage/retrieval of data on/from tape. It has read/write heads. As tape ribbon passes under the read/write heads. it can read/write data from/to the tape by giving suitable commands (read or write) to the tape drive. Like in the case of an audio tape recorder or video cassette recorder, we have to first load a magnetic tape reel, cartridge, or cassette on to a tape drive attached to a computer for processing. The magnetic tape is on-line once we load it on a magnetic drive. The computer can now use it for storage or retrieval of data. When processing is complete, we remove the tape from the tape for off-line storage (away from the computer system), and data on it becomes inaccessible until we load it again on the tape drive.
This is a removable storage device mainly used for backing up data. It is similar to a Zip Drive, but instead of Zip disks, it uses small tapes. The drive acts like a tape recorder, reading data from the computer and writing it onto the tape. Since tape drives have to scan through lots of tape to read small amounts of scattered data, they are not practical for most storage purposes. That is why they are used almost exclusively for data backup. The benefit of tape drives is that they typically have large capacities for storing data, for a lower cost than hard drives similar in size. Also, multiple tapes can be used to make incremental backups (daily, weekly, monthly, etc.), which is much cheaper than using multiple hard drives.