One of the most neglected types of removable-drive technologies is the magneto-optical (MO) drive. Introduced commercially in 1985, magneto-optical drives are now available in capacities exceeding 9GB.

Two sizes of magneto-optical media and drives are available for desktop computers: 3 1/2'' and 5 1/4''. The 3 1/2'' drives have capacities up to 2.3GB, and the 5 1/4'' drives have capacities up to 9.1GB. 12'' MO drives are also available for enterprise systems.

Originally, magneto-optical drives were strictly WORM (write once, read many) drives that produced media that could be added to, but not erased. WORM drives are still available on the market, but for desktop computer users, read/write MO drives are preferable.

Magneto-Optical Technology

At normal temperatures, the magnetic surface of an MO disk is very stable, with archival ratings of up to 30 years. One surface of an MO disk faces a variable-power laser, whereas the other surface of the disk faces a magnet. Both the laser beam and the magnet are used to change the data on an MO disk.

The "optical" portion of an MO drive is the laser beam, which is used at high power during the erasing process to heat the destination area of the MO drive to a temperature of about 200° Celsius (the Curie point, at which a normally magnetic surface ceases to be magnetic).

This enables any existing information in that area to be erased by a uniform magnetic field, which doesn't affect the other portions of the disk that are at normal temperature.

Next, the laser beam and magnetic field are used together to write information to the location by applying high power to the laser and applying a controlled magnetic signal to the media to change it to either a binary 0 or 1.

During the read process, the laser is used at low power to send neutrally polarized light to the surface of the MO disk. The areas of the MO disk that store binary 0s reflect light at a polarization angle different from those that store binary 1s. This difference of one degree is called the Kerr effect.

In older MO drives, the erase and write process involved two separate operations, but most recent MO drives starting with the Plasmon DW260 of 1997 use the LIMDOW method (light intensity modulated direct overwrite) for a single-pass operation with some media types.

LIMDOW drives use magnets built into the disk itself, rather than separate magnets as in older MO drives. LIMDOW drives are fast enough to store MPEG-2 streaming video and make achieving higher capacities easier. Most internal MO drives connect to SCSI interfaces. ATAPI/IDE models are sold by some vendors but aren't as easy to find, and external MO drives are available in SCSI, USB 2.0, and IEEE-1394a interfaces.

Comparing MO to "Pure" Magnetic Media

Compared to most high-capacity removable-media drives, MO drive hardware is more expensive (especially in the 5 1/4'' media size), but media costs are far less per MB, durability is far better, and performance is as good or better than the 200MB-or-under class magnetic removable-media drives.

The use of SCSI interfaces for most models was a drawback when MS-DOS/Windows 3.1 were the leading operating systems, but Windows 9x/Me/NT/2000/XP have much easier SCSI installation processes, and SCSI interfaces have dropped in price (and are included with some internal drives).

IEEE-1394a and USB 2.0 interfaces available on some models allow the easiest installation process of all for external drives. If you can afford the high initial cost of the 5 1/4'' media MO drives, you'll have a fairly fast, durable, long-term storage solution that's also a good storage area for works in progress.

Selecting a Removable Drive

When shopping for a removable drive, keep the following in mind:

• Price per megabyte of storage. Take the cost of the drive's cartridge or disk and divide it by the storage capacity to see how much you are paying per megabyte of storage. This difference in price becomes quite apparent as you buy more cartridges or disks for the drive.

Don't forget to factor in the cost of the drive itself if you are trying to decide which removable-media drive to buy! If you plan to use removable storage for temporary data storage only, as with flash memory devices, the cost per megabyte is a less important factor than if you plan to leave data on a cartridge or disk for long periods of time.

• Access time versus need of access. The access and data transfer speeds are important only if you need to access the data frequently or quickly. If your primary use is archiving data, a slower drive might be fine. However, if you plan to run programs off the drive, choose a faster drive instead.

• Compatibility and portability. Opt for an external SCSI, IEEE-1394a, USB, or parallel port solution if you need to move the drive between various computers. USB is the lowest-cost and friendliest solution because it's built into recent systems and supports hot-swapping.

Also verify that drivers are available for each type of machine and operating system you want to use with the drive, and consider whether you need to exchange disks with other users. The Iomega Zip disk and Imation LS-120 SuperDisk drives have become standards for magnetic removable-cartridge media.

However, if you don't want the expense of buying a separate drive for each machine, USB keychain storage devices can perform the same tasks and don't need a separate drive for each computer. For some users, this might be the most important factor in choosing a drive.

• Storage capacity. For maximum safety and ease of use, the capacity of your storage device should be the largest available that meets your other requirements. Digital camera users, for example, will want the largest possible flash or disk storage supported by their cameras to allow more photos or higher-quality photos to be stored. Desktop and notebook computer users will want the largest drives possible for data backup or program storage.

• Internal versus external. Most users find external USB drives the easiest to install; additionally, they give you the option of using the drive on several systems. Internal drives are usually faster because of their ATA or SCSI interfaces and are more cleanly integrated into the system from a physical perspective.

• Bootable or not. Most systems dating from 1995 or later have a BIOS that supports the Phoenix El Torito standard, which enables them to boot from CD or DVD drives. Most also support the ATAPI/IDE version of the Imation LS-120 SuperDisk as a bootable device.

However, those drives are obsolete and difficult to find. Some systems support ATA Zip drives as a bootable drive, but Zip drives are a proprietary format and incompatible with standard 3 1/2'' floppy disks.