The distinctive design of the DAT tapes read/write head is what allows it to back-up huge amounts of data onto a small tape cartridge about the size of a matchbox. The mechanism is a rotating cylinder with four heads spaced 90 degrees apart around the circumference of the drum.
Two of these heads are Write heads (opposite each other) and the other two heads between them are Read heads. The Read heads can verify data as it is written by the Write heads.
The cylinder spins at 2,000 revs per minute while the tape is passed across the head at about one third of an inch per second, in the opposite direction to the rotation of the cylinder.
During the time each write head is in contact with tape it writes about 128K of data and error correction codes to the tape. This data is stored in a write buffer in the tape drive. Because the cylinder is tilted in relation to the direction of travel of the tape the head encounters one edge of the tape as it starts to write and moves diagonally across the tape to the other side, writing a narrow track that is about eight times as long as the width of the tape. The first read head then comes into contact with the tape and reads the data put down by the first write head, verifying against the data held in the write buffer. If the data read from the tape is correct the drive flushes the buffer and requests another "buffer full" of data from backup software. If the data is incorrect the same data is written again, a process that continues until the data in the write buffer is stored on the tape correctly.
As the cylinder rotates it brings the second write head into contact with the tape and this head then writes a write buffer full of data onto the tape. This is where things get a bit tricky. In order to increase the write density the second head writes data at an angle of 40 degrees to the track formed by the first write head, making a criss-cross pattern that overlaps the track produced by the first head. The overlapping data packs more information per inch of tape. This track written by the second write head is not misread by the second read head because the two different pairs of read/write heads have different polarisation and the read heads only read data from the correct tracks. The process goes on and on writing and reading from the first pair of heads then the second pair of heads.
A directory of all the files on the tape is stored on the tape and a second copy of this is usually kept on the disk that has been backed up. This is done to speed up access to the tape sometimes by not having to look up the directory information on the tape.
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