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Removable Media

For the purpose of this module removable media includes external devices such as CD-ROM, tape, and floppy drives. These are also referred to as storage devices as they are used to hold program data and other information.

Each vendor provides documentation regarding which removable media devices are known to work with its system. This generally means that the appropriate device drivers are a standard part of the operating system. Other peripherals may work with a given system, but the device drivers will have to be added. Additionally an unsupported device may not be compatible with other utilities on the system.

Selecting a CD-ROM or floppy drive is a fairly straightforward process. There are, however, a variety of tape drives available for different tape formats and capacities.

Quarter Inch Cartridge. This acronym is also reported to stand for Quarter Inch Committee. There are standards for QIC drives so that drives produced by different companies are interchangeable. Additionally hardware and software development standards for QIC subsystems allows a quarter-inch data cartridge recorded on one system to be used with another compatible system. These drives are made to work with differing lengths and can copy at different densities. The last three digits of the model number refer to the drive capacity in MB. The most common cartridge tapes are QIC-150, QIC-320, and QIC-525. Usually QIC drives can read tapes written in a less dense format, although they may not be able to write to them. QIC drives cannot backspace.

4mm DAT (Digital Audio Tape)
The DAT standard for data storage is DDS (Digital Data Storage). However, the drive and the media are similar to audio DAT. DAT tapes are about the size of a match box and are the smallest magnetic media available. Each tape holds around 2 GB of data. This can be increased up to 8 GB with compression.

8mm DAT (Digital Audio Tape)
These drives are sometimes referred to as "Exabytes" as they were the first company that made them. They record to standard 8mm videotapes. Original format holds 2 GB of data while the newer format will hold 5 GB. Compression on some drives can push the capacity up to 10 GB. 8mm tape drives contain drive mechanisms that are know for drifting out of alignment every few years.

DLT (Digital Linear Tape)
These drives have a storage capacity of up to 40 GB per tape with compression. DLT drives are designed with a dual-channel read/write head which allows them to record data twice as fast as other drives at a given speed and recording density.

An additional note on floppy drives and disks: there are utilities available that allow Unix to read DOS or Mac floppies. Mtools is set of public domain programs that enables Unix to manipulate MS-DOS files. SGIs will read both DOS and Mac floppies. Under IRIX, the mediad daemon determines the format of the floppy disk and if it is Mac or DOS, mounts the filesystem on the default mount directory. From this directory regular Unix commands can be used to access the files.

Adding removable media to a system involves three basic steps:

  1. Powering down the system.
  2. Installing the device. This includes connecting it physically, setting the SCSI ID, and making sure the proper device drivers are present.
  3. Powering the system back up.
The specific details vary from system to system.

Up to six SCSI devices may be daisy chained together. This means that rather than attaching peripherals to the computer itself they may be hooked up to each other. The last device in the chain must be terminated with an active terminator. This terminator should come with the device.

Each SCSI device must have a unique SCSI ID or address. This is a number between 0 and 7. One of these IDs is reserved for the controller, either 0 or 7. To find out which SCSI addresses are available:

Unfortunately there is no single solution for getting this information under Linux. The command dmesg may work. If Linux is running with a fairly new kernel, cat /proc/scsi/scsi may work. There is also a file in the /proc/scsi directory for the SCSI host adapter that the system is using. Depending on which driver is in use, this file may also contain the relevant information.

The SCSI ID can be set with a switch, dial, or jumper as per the instructions that come with the device. The device should also come with instructions on how to physically attach it to a system, including which cables are needed, etc.

For details on the installation process for each platform please see:

Terms used: QIC, DAT, compression, DLT, jumper, SCSI.

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