What goes around comes around. High-capacity removable storage, typified by the runaway success of the Iomega Zip and, to a lesser extent, Jaz drives, may be the flavour of the month right now. But cast your mind back to 1981 and the launch of the original IBM PC.

This PC had no fixed storage at all ? it only came with removable storage, in the shape of a pair of 5.25in Tandon floppy drives, with the awesome capacity of 160Kb each. And users still queued up to buy it in droves. Shortly after that, Al Shugart managed to shoehorn a hard disk into the 5.25in form factor. And since then PCs have generally been sold with both fixed and removable drives.

The development of removable storage has only really come to a head over the past couple of years. Sure, there have been developments over the intervening years but they have singularly failed to bridge the ever-widening capacity gap between the typical removable disk, the floppy, and the typical hard disk.

Think back ? the first hard disks had a capacity of 10Mb and while it may have been a pain, it was possible to back up the entire hard disk on to 28 360Kb floppies. Three years later, in 1984, IBM introduced the PC/XT replacement, the AT and this featured 5.25in high-density disk drives capable of storing 1.2Mb.

ATs shipped with 20Mb hard disks, so just 17 backup floppies were required. Back then, the ratio of removable to fixed storage capacity was manageable.

That same year, Apricot and Hewlett Packard launched PCs that featured a revolutionary 3.5in disk drive made by Sony. This had a 720Kb capacity ? nothing to write home about today, but it was totally enclosed in a stiff plastic sleeve, with a sliding cover. Two years down the road, its capacity was doubled to 1.44Mb by the simple expedient of doubling the number of sectors per track.

So far, so good. However, for the next 10 years or so the capacity of the floppy remained stuck at 1.44Mb. In the meantime, hard disk capacities have rocketed to a point where a new PC will typically have a hard disk with roughly 1,500 times the capacity of the floppy drive it comes with. Today, the ratio of removable to fixed storage is wildly out of kilter.

It?s true there have been several attempts to break the 1.44Mb barrier, but they have amounted to naught. Back in 1991, IBM tried to impose a 2.88Mb floppy standard, using expensive barium-ferrite disks but that died a death.

Both Iomega and 3M had another go in 1993 with the 21Mb floptical disk. But the floptical disk never took off ? it was just too dear and too small. So, until quite recently, the choices for removable storage were extremely limited ? floppies were too small, tape drives were clumsy and optical solutions were exotic and expensive. The best alternatives were removable cartridges from Iomega and Syquest, but both used proprietary formats and were dear.

Things are very different now ? the market for removable drives has widened considerably. This is largely due to the high profile generated by one company, Iomega, and its two removable drives, the 100Mb Zip and the 1.2Gb Jaz drive, which were launched in the UK in 1996.

These drives were so attractively designed, aggressively marketed (as ?personal storage?) and so competitively priced that other manufacturers had to take notice.

New technologies have also recently emerged or have become affordable, such as CD-Recordable (CD-R) and PD Phase Change. As a result, the choice of removable drives is now wide, offering a range of storage capacities, beginning at 100Mb and topping out at several gigabytes.

So why has removable storage become so important recently? And precisely what can it be used for? The former is easily answered ? both data and program files have ballooned in size, due to the impact of things like Windows and multimedia. Once upon a time a 1.44Mb floppy could comfortably accommodate your data files, but not any more.

Although they all function differently or employ differing technology, all removable-media drives have many things in common besides the removability of the media. Increasing capacity by using additional cartridges is much easier than buying and fitting a high-capacity hard drive. Cartridges are durable and are easier to transport or store than entire drives. And although no removable-media drive is quite as fast as a hard drive, there?s been a steady improvement in speed as the new technologies have been developed.

Today?s high-capacity removable storage devices are most commonly used as personal backups and archives and for transporting large files. They also offer unlimited storage, in that a new disk can be inserted after one fills up.

Do bear in mind, though, that you?ll have to buy a few disks before such a drive?s cost per Mb drops to that of ordinary hard disks ? for example a 1.2Gb Jaz sells for about #250 and an additional cartridge costs #60, compared with a 1.2Gb hard drive which goes for around the #110 mark.

There are basically three types of drives: external drives that plug in to a parallel or Scsi interface, and internal drives that either use a Scsi or IDE interface, though the latter interface is pretty rare at the moment ? only the Syjet and the LS-120 support IDE.

The easiest drive to install is the parallel port version. This makes it easy to share one drive between several desktop PCs. It also lets you use the drive with a notebook. The only downside is that the parallel port can act as a bottleneck, slowing down the drive?s data transfer rate, although modern PCs, fitted with ECP parallel ports, are capable of excellent throughputs and would not impinge on slower devices, such as the Zip or LS-120.

Internal drives, at the moment, tend to use a Scsi interface, which is fine if you have a Scsi card already fitted, which makes installation as simple as plugging the drive into a spare connector or the Scsi ribbon cable into the back of the drive.

External drives with Scsi interfaces are just as simple to install. You?ll get superb data transfer rates by going down the Scsi route, but if you haven?t got a Scsi host adaptor, to give the card its proper name, then you?ll have to buy one (and they don?t come cheap) and then install and configure it.

Super Floppy ? Iomega Zip Drive

Cost: #115 (drive); #12 (disk)

Capacity: 100Mb

Maximum sustained data transfer rate: Approx 1.4Mbps

Average access time: 29ms

Pros: In widespread use. Inexpensive, portable.

Cons: Moderate performance.

The Zip drive is a bit like an oversized floppy and shares with that old technology the benefits of portability and low cost. It?s easy to install and also comes with good software both for Windows 95 and Windows 3.1x.

One great feature is its Guest software driver, which lets you use a Zip drive on any computer system without installing all the Zip drive files on a hard drive. You simply install the drive, boot the system, put the Zip drive install floppy in a floppy disk drive, type: GUEST and then let the driver handle the rest.

The Zip drive has proved extremely popular and Iomega claims to have sold over 10 million drives in the first 12 months. PC manufacturers, including Gateway, are offering Zip drives as options and the long overdue slimline notebook version is promised for the summer.

Hard disk ? Syquest EZ Flyer Drive

Street price: #165 (drive), #20 (disk)

Capacity: 230Mb

Maximum sustained data transfer rate: Up to 2.4Mbps

Average access time: 13.5ms

Pros: Fast and affordable drive, small enough for travel.

Cons:

The cartridges don?t automatically eject.

Syquest drives have been around for yonks and the 3.5in EZ Flyer drive is based on proven hard drive head and media technology. It?s cheaper, holds more data and is faster than the Zip, but it is a bit harder to install and use.

The EZ Flyer is available in parallel, Scsi and IDE configurations. Once installed, the EZ Flyer behaves just like a small, fast hard disk. EZ Flyer can read, write and format both EZ Flyer 230 and EZ135 cartridges.

Hard disk ? Iomega Jaz Drive

Street price: #250 (internal drive), #60 (disk)

Capacity: 1.2Gb

Maximum sustained data transfer rate: Approx 6.5Mbps

Average access time: 12ms

Pros: Very fast drive. Affordable, high-capacity media.

Cons: Dearer than ordinary hard disks.

The Jaz is the fastest but not the largest capacity removable drive commonly available today. Its 1Gb-plus capacity makes it possible to back up your main hard disk in one go, in just a matter of minutes.

In fact, the Jaz drive is almost as fast as that hard disk inside your computer. It?s available in both external and internal Scsi versions plus an external parallel port model. It has proved very popular in the marketplace.

Magneto-optical ? Fujitsu Dyna MO 640

Street price: #375 (drive); #35 (disk)

Capacity: 640Mb

Maximum sustained data transfer rate: 2.3 to 3.9Mbps

Average access time: 35ms

Pros: Very durable media. Universal compatibility with other 3.5in optical drive formats.

Cons: Relatively slow drive. Scsi only.

Magneto-optical technology (MO) was very nearly killed off by the Zip but a new generation of drives, spearheaded by Fujitsu, which are both faster and cheaper, have breathed new life in to the format. MO disks are much the same size as 3.5in floppies but about twice as thick.

Although its data transfer rate is good (it?s a Scsi device) its average access time isn?t too good, compared with the Jaz or Syjet, say. One downside of this technology is that a write is a two-pass operation and this makes it a slow process. The very latest MO drives now feature faster, one-pass write technology.

MO disks are also very durable and are often rated at an average life span of 30 years (probably far longer than you are likely to have the drive on which to read the disk?s contents), and are rated for an unlimited number of write cycles, making them a good choice for archiving duties.

Hard disk ? Syquest Syjet

Street price: #315 (internal drive), #75 (disk)

Capacity: 1.5Gb

Maximum sustained data transfer rate: 3.7Mbps to 6.9Mbps

Average access time: 12ms

Pros: Very fast drive. Affordable, high-capacity media.

Cons: Dearer than ordinary hard disks.

The Syjet is Syquest?s belated attempt at a Jaz basher. Although its performance is no better than the Jaz, it has a slightly bigger capacity, though the disk cartridges are 25 per cent dearer. However, it has done relatively well in product reviews and has garnered a number of ?best buy? and ?editor?s choice? awards since its release. Interestingly, it?s available in IDE or Scsi versions.

Hard disk ? Xyratex MaxIT

Street price: #200 (internal drive), #40 (disk)

Capacity: 540Mb

Maximum sustained data transfer rate: Approx 6.5Mbps

Average access time: 12ms

Pros: Syquest compatible. As fast as the Jaz ?

Cons: ? But not as big

Xyratex used to be the division of IBM that built hard disks down at Havant, so while you may not have heard of this company, it has an awful lot of hard disk expertise. Like the EZ Flyer, the Max IT takes its visual clues from the Iomega house style, with bright yellow and blue external cases.

The Max IT uses the latest 3.5in Syquest/MCD removable disk technology, which gives it both good capacity ? half a gigabyte ? and excellent performance. It is also backwardly compatible with Syquest 270Mb cartridges.

It?s available in both internal and external versions, Scsi and parallel. A similar product also appears under the Nomai label.

Super Floppy ? 3M LS-120

Cost: #160 (drive); #15 (drive)

Capacity: 120Mb

Maximum sustained data transfer rate: 1Mbps approx

Average access time: 100ms

Pros: Good capacity, backwardly compatible with old floppies

Cons: Slow data transfer rate. IDE only.

Iomega actually developed the LS-120 technology but decided it had less to offer than Zip and so sold it off to 3M. Named after the laser servo technology it employs, LS-120 technology places optical reference tracks on the disk that are both written and read by a laser system.

These servo tracks are laid on the disk, and are tracked by a laser to allow for much finer head movement, with the result that an LS-120 disk surface can hold 2,490 tracks per inch (tpi) compared with just 135tpi on a standard HD floppy.

After a very slow start in late 1996, the LS-120 now has fairly widespread support from PC manufacturers. However, these still tend to offer it as an option rather than as a replacement for the standard 1.44Mb drive. As well as being backwardly compatible with old 3.5in floppies, the LS-120 can also be set up as the A: drive and so is bootable under Windows 95, provided the Bios supports it (the same is also true of the Zip).

The Zip has a noticeably faster data transfer rate but that?s because it spins four times faster. Some maintain this shortcoming isn?t so important ? it depends on the size of the files you?ll want to copy.

Although not quite so affordable as the drives above, two other removable storage technologies are worth mentioning. The first is CD-R which lets you ?burn? your own CD-Roms. CD-R drives now cost less than #500 and blank disks cost from about #5, which makes CD-R a cheap, if slow, way of storing data.

Also new on the scene is the PD Drive from Panasonic. Using the new phase change technology, the #285 PD Drive is a four-speed CD-Rom drive that doubles as a rewritable 650Mb PD drive. Disks are reasonably priced at #20.

On the horizon is the N.hand, a new, ultra-compact drive from Iomega. Little bigger than a book of matches, the N.hand disk will have a 20Mb capacity.

Previewed at Cebit 97, the new disk format is set to rival expensive flash memory cards currently employed in PDAs, digital cameras and other handheld devices ? the new disks will sell for $10 or less.

Iomega is also working on plans to make the new N.hand disk technology compatible with future versions of its Zip drive.