(Thanks to Yehavi Bourvine for the help and part of the information,
  and to Glenn Everhart for the very informational comments) 

 Magnetic tapes and cartridges are used to store very large amounts of
 data that will not be used frequently. Locating a specific piece of 
 data on a magnetic tape is slow, as tape velocities are relatively low
 and you have to read all information preceding it.

 Magnetic media types (tapes and cartridges)
 Cartridges (of the same size) with shorter tape length hold less data but 
 may be more reliable because the magnetic tape is thicker and the influence 
 of one tape winding on the others is less due to the larger separation of 
 the magnetic coating.

 A relatively new innovation is MRC (Media Recognition System) found usually 
 on 4mm cartridges. Cartridges with MRC have the relevant tape specifications 
 written in the beginning of the tape so the tape drive can read it, identify 
 the tape type and use the tape in the correct way.

 4mm DAT tapes are considered by some people to be more reliable than 8mm, 
 in spite of the fact that 8mm recorded on the medium bit density is less 
 than 4mm. 

 However, much of the problems folks have had with 8mm, may have been due to 
 the lower physical quality of some drives (especially older ones), and has 
 also been driven by the fact that video grade tapes are easy to use and a 
 lot more easier to get (and a LOT harder on the drives) than data grade tape. 
 Some video grade 8mm's are said to damage the head after 3 or 4 uses!

 4mm audio grade tape is as hard to find as data grade so folks use the data 
 grade stuff more often. 

   Comparison of different magnetic media types

  Type    | Tape   | Capacity   | log | Record  | Density | Record | Operation
          | dims   | /compacted | BER | method  | (bpi)   | format | mode
 4mm DAT  |  60m   | 1.3/~2.6GB | 15  | Helical | 1869    | DDS    | Streaming
          |        |            |     | scan R  | tr./in. | ANSI   |
          |  90m   | 2.0/~4GB   | 15  | Helical | 1869    | DDS    | Streaming
          |        |            |     | scan R  | tr./in. | ANSI   |
          | 120m   | 4/~8GB     |     |         |         |        |
          |        |            |     |         |         |        |
 8mm      | 60ft   | 2.3GB      |     |         | 5400    |        | Streaming
          |        |            |     |         | bpi     |        |
 8mm      | 112m   | 5/~10GB    | 17  | Helical | 8500    | Vendor | Streaming
          |        |            |     | scan    | bpi     | specif |
 TK25     | 0.25"  | 60MB       |     |         | 8000    |        | Streaming
          |        |            |     | 10 Tr.  | bpi     |        | 
 TK50     | 0.5" x | 95MB       |     | Serpent | 6667    |        | Streaming
          | 600ft  |            |     | 22 Tr.  | bpi     |        | 
 TK70     | 0.5" x | 296MB      |     | Serpent | 10000   |        | Streaming
          | 600ft  |            |     | 48 Tr.  | bpi     |        |
 QIC      | 0.25"  | 60MB       |     |         |         |        | Streaming
          | 425ft  |            |     | 9 Tr.   |         |        |
 QIC      | 0.25"  | 150MB      |     |         |         |        | Streaming
          | 700ft  |            |     | 18 Tr.  |         |        |
 QIC      | 0.25"  | 320,525MB  |     |         | 1000    |        | Streaming
          |        |            |     |         | bpi     |        |
 IBM-3480 |        | 200MB      |     | Linear  |         |        |
 TA90     |        |            |     | 18 Tr.  |         |        |
 6150     |        | 150MB      |     |         |         |        |
 6250     |        | 250MB      |     |         |         |        |
 Typical  | 0.5" x | 145, 40MB  | 11  | Linear  | 1600 PE |        | Start/Stop
 reel     | 2400ft | GCR, PE    |     | 9 Tr.   | 6250 GCR|        | Streaming
          | D10.5" |            |     |         | bpi     |        |

 Tape reel and magnetic cartridge drives
 Classical tape reel drives can accelerate and decelerate rapidly, 
 for example they can read a record, decelerate to a stop in the 
 Inter-Record Gap and accelerate again and read the next record 
 (see below for an explanation of these terms).

 It is cheaper to have STREAMING DRIVES, these have simpler mechanisms 
 and weaker motors, so they need more time (and tape length) to accelerate 
 to reading speed and decelerate to a stop. Streaming drives perform
 well only on long reads and writes, if the operating system can keep up
 with the read/write rate.

 Tapes are used mainly for data backups and physical transfer of large 
 data files, for these purposes streaming drives are adequate.

 Methods of data encoding
 Tapes are coated with a material that can be easily magnetized in different 
 directions by the magnetic field produced by the drive's head. The same
 drive head may be used to read the magnetization direction.

 Drive heads cannot actually detect the magnetization direction, but only
 changes in this direction. This problem is solved by having the drive 
 hardware 'remember' the direction in the previous time unit, if a direction 
 transition occurred the new direction has reversed, if not it is the same.

 Of course we can't know the initial magnetization direction, we can 
 arbitrarily call it UP or DOWN as we like, it doesn't matter because all
 coding methods are based on direction transitions, not directions.

 Another fundamental problem at the hardware level is to determine while 
 reading when each bit starts and ends, so we can properly sample and 
 decode its value. 

 The tape speed and recording density are supposed to be constant, so 
 bits are read at a (more or less) constant rate, but in practice we 
 can't depend on that, and we must 'resynchronize' frequently. 

 The synchronization problem may be solved either by having separate
 synchronization pulses on a special track, or incorporating them into
 the data track.

    NRZI (Non Return to Zero - Inverted)
    In this method the magnetization direction is reversed for 1(binary),
    and remains constant for 0(binary). NRZI obviously needs a separate 
    synchronization track, e.g. if a series of 0(binary) is recorded,
    the magnetization direction will remain constant, and we need some 
    means to count the 0(binary) bits.

    In NRZx methods we keep on a separate track 'clock' (synchronization) 
    pulses - transitions made every time unit. Every clock pulse we check 
    if a transition occurred and compute the bit value accordingly.

    Typical density of NRZI recording is 800 bits per inch

    Phase encoding (PE)
    Also called biphase-mark, Harvard, Manchester or split-frequency system.
    Here the data contains the clock pulses, a direction transition occurs 
    every bit. We use the direction transitions as our basic signs, e.g. an 
    UP/DOWN transition may be our 1(binary) and a DOWN/UP transition will be 
    0(binary). If there is no direction transition, we get another useful 
    sign that we will call NO-VALUE (we will not be able to count such 
    successive signs). 

    By the way, between any two consecutive transitions of the same type we 
    will get an extra non-data transition, that artifact can be ignored by 
    the drive circuitry.

    Typical density of phase encoding is 1600 bits per inch.

    GCR (Group Coded Recording)
    Typical density is 6250 bits per inch.

 The internal structure of tapes
 The internal data structures of tapes are very robust, and can withstand 
 to some degree controller, tape head or magnetic media errors.

 We will think of a written tape as nine long sequences that run side by 
 side and are composed of the three signs: 0, 1 and NO-VALUE, eight of the 
 sequences convey the information and the ninth is used as a check 
 (odd parity check). In addition there is the BOT mark (Beginning Of Tape) 
 a little after the beginning and the EOT mark (End Of Tape) a little 
 before the end. 

 The BOT and EOT marks are not written on the tape using the three signs 
 but are pieces of photo-reflective tape.  

 A character is eight 0 or 1 signs, one from each sequence. A series of 
 characters delimited on both sides by a long (about 0.6 inch) series 
 of NO-VALUE signs is a record. The delimiting region is called the IRG 
 (Inter Record Gap).

 "Mixing" a record and an IRG creates the important TAPE-MARK, that serves 
 as a general delimiter at the hardware level. A tape-mark is created when 
 some of the nine sequences are composed of NO-VALUE signs only, and some 
 are constant having value of 0 or 1. The drive hardware can easily create 
 or recognize this state.

 A series of records delimited on both sides by a tape-mark is a file, the 
 IRG's near the tape-marks are then made longer. 

 Two consecutive tape-marks denote a "logical" end-of-volume. You can have 
 more than one "volume" on the tape, but you have to do some programming to 
 pass between them (?).

 Labels are records (length = 80 characters), they are separated from other 
 records by a tape-marks. Labels are used to keep security and structure 
 information. There are several kinds of labels: Volume label, File header 
 labels and file trailer labels etc. 

    Scheme of a tape structure 
       BOT mark
       VOL1 label
       HDR1 label  ----+
       HDR2 label      |
       Tape-mark       |
       Data record     |---- the first file
       ...........     |
       Data record     |
       Tape-mark       |
       EOF1 label      |
       EOF2 label      |
       Tape-mark   ----+
       ........... ----+
       ...........     | unwritten tape area
       ........... ----+
       EOT mark

 The tape structure is actually more complicated because:

    1) More labels can be present.
    2) The files can be continued to another tape.
    3) Data can be written behind the EOT.

 Using tapes (general)
 The basic tape operations are:

    Drive allocation    Tell the operating system it should not let
                        other users do anything with that tape drive

    Initialization      Build the basic data structures on the tape, all
                        previous data on the tape will be inaccessible!
                        Similar to formatting a PC diskette

    Mount               Connecting the tape drive to the file system,
                        the system software that controls all accesses
                        to disk and tape drives.

    Rewind              Spinning the tape to the beginning

    Unload              Opening the drive door and ejecting the cartridge

    Write               Made with FORTRAN's WRITE statement

    Read                Made with FORTRAN's READ statement

    Deallocation        Tell the operating system you don't need
                        the tape drive anymore

    Dismount            Cancel a mount command, disconnect the tape drive
                        from the file system.

 Data protective measures
 The safety switch on a cartridge back side, will prevent writing on the 
 cartridge, when in the write-protect position.

 The integrity of each data byte is guarded by the parity bit. Professional 
 tape drives and utilities like UNIX/TAR and VMS/BACKUP and VMS/DUMP use 
 better schemes like CRC (Cyclical Redundancy Code) and Reed-Solomon ECC 
 (Error Correcting Code) that has far better ability to correct errors.

 The volume label on VMS gives some protection against writing on the wrong
 cartridge or tape. When the tape is initialized you have to supply an 
 alphanumeric string that is written into the volume label. Other tape 
 operations may require that you respecify that string, it is compared with 
 the string written in the volume label and if found different, the operation 
 is aborted. 

 Using tapes on VMS

    Device names
    Tape drive names on VMS are composed of:

       Allocation-class    Optional, 1-2 digits enclosed by '$' signs
       Type prefix         The letters 'M', 'T'
       Type                K - SCSI
                           U - Generic
       Controller id       A - internal IO bus,  B - external IO bus
       Device number       0,  1,  2,  3,  ...
       Placeholder         If device number is not zero, add '00'
       Colon suffix        ':'

    For example let's analyze the device name  $4$MKB500:  

       The allocation class is 4, this is site-specific information
       The 'M' says it's a magnetic tape drive 
       the 'K' means a SCSI drive 
       'B' says the controller is external to the computer box 
       Device number 5 means the SCSI ID is 5

    An important note:  

       The device naming convention may change as VMS is now
       supporting a lot more similar devices simultaneously.

    Foreign and structured tapes and mounting modes
    VMS supports the ANSI standard for tapes, and provides an extension
    to that standard. Tapes initialized on VMS contain some more data 
    structures that keep security and structure information.

    Using the additional data structures it wrote on the tape, VMS let
    you use the tape just like an ordinary disk, albeit very slow.

    A tape without the additional VMS data structures is ANSI conforming,
    in VMS terminology it's called a 'foreign' tape, and you can mount
    it in 'foreign' mode with the  MOUNT/FOREIGN  command. 

    Tapes written on other operating systems are usually ANSI conforming 
    and can be read in foreign mode.

    A tape initialized by VMS (contains the additional data structures),
    can be mounted either in foreign mode using  MOUNT/FOREIGN, or in
    'structured mode' without the /FOREIGN qualifier. 

    The following table summarizes the practical aspects of the foreign 
    and structured mounting modes:

         FOREIGN MODE                    STRUCTURED VMS MODE
        --------------------------      --------------------------------
        You don't have to know the      You need the label (if you don't 
        volume label to mount it.       know it, mount the tape /FOREIGN 
                                        then VMS will tell you the label, 
                                        dismount and mount again!)

        You can't do a DIR, and can     You can do a DIR with all switches,
        COPY only to another foreign    Unrestricted COPY 
        mounted tape.

        Can use BACKUP                  Cannot use BACKUP

        Can use DUMP with only the      Can use DUMP with complete file
        drive name                      specifications (and wildcards)

        SET MAGTAPE works and you       SET MAGTAPE doesn't work, but you
        can change drive properties     don't need it either!
        and do some tape operations

    Relevant commands

    *ALLOCATE drive-name

    *INITIALIZE drive-name volume-label

     MOUNT                     drive-name volume-label logical-drive

     SET MAGTAPE               drive-name

       * doesn't require knowing the tape label

    Common error messages related to tapes
    VOLUME IS WRITE LOCKED   The safety switch on the cartridge is in
                             the wrong position, and the cartridge is
                             now write protected.

    MEDIUM IS OFF LINE       The drive door is open(?).

    DRIVE IS OFF LINE        Something happened to the tape drive.

 Reading and writing

 Using tapes on UNIX

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