The slow but irresistible move from HDD (Hard Disk Drive) to SSD (Solid State Drive) is already under way and is sure to accelerate over the next few years. Give it three years - five at the outside - and you'll be hard-pressed to find a new laptop that uses a conventional hard drive.

Memory manufacturer Crucial has jumped into this market with two models of SSD that are direct replacements for the 2.5-inch hard drive in your laptop. At present the capacity of SSD is relatively small compared to HDD and if you buy a Crucial drive it will cost you £10 per Gigabyte. By contrast a popular 200GB HDD will cost you £80, which works out to 40p per GB, and it's easy enough to get your hands on a 250GB drive for £50 which is 20p per GB.

Don't be discouraged, though, as other makes of SSD are considerably cheaper with prices that dip under £3 per Gigabyte. Happily Crucial has added another element to the equation that makes its SSDs more interesting. This is the SK01 external storage kit which sells for £29.36 inc. VAT and which transforms the SSD into a portable storage unit.

The first part of the kit is a slender metal housing. Remove two screws; pop off the cover, insert the SSD and then replace the cover and you now have the choice of a USB connection in addition to SATA.

You may find that your PC or laptop can't supply enough power over a single USB connection so Crucial supplies a double-head USB cable in the package that can plug into two ports simultaneously to supply the necessary juice.

The next goodie out of the box is a drive bay which fixes into a floppy bay in your desktop PC's case. You connect a SATA cable to the rear of the bay and hook up a power adapter that is supplied and can then pop the SSD in and out, much like we used to do with Zip drives. Crucial supplies a protective carrying case to keep your SSD safe when you're on the move.

Crucial also supplies an adapter so you can install the drive bay in a 5.25-inch optical drive bay, so we fully expect that you'll be able to install the kit in any PC that has a spare drive bay.

The question is whether you should bother with the Crucial hardware. You can buy a 32GB USB memory key for £75 that will connect to any PC or laptop on the market so the hassle and expense of the Crucial system is both unnecessary and unwelcome if you're looking for external storage.

Things are slightly different if you plan on using the SSD as an internal drive to replace a conventional HDD. The most obvious change is the complete silence of the SSD which is fairly startling compared to even the quietest hard drive, but that's the limit of the benefits.

We expected the Crucial SSD would be extremely fast compared to a hard drive, as solid state memory doesn't have the mechanical limitations of a hard disk, but this was not the case. In PCMark05 the test results were quite dreadful and merited further investigation. Transferring files between the Crucial SSD, a Hitachi 7K1000 and a WD VelociRaptor showed that read speeds from the Crucial SSD were fast while write speeds were very slow.

The HDTach utility put some numbers on the problem and showed that random access times were incredibly fast at 0.4ms, whereas most hard drives have a response in the range of 7ms to 13ms. The other oddity is that burst speeds for the Crucial are very low indeed at 113.7MB/second as most drives can manage 200MB/second or more.

We're excited about the introduction of Solid State Drives as we love the prospect of silent drives that are blisteringly fast, silent in operation and immune to damage from bumps and knocks. Unfortunately the 32GB Crucial SSD fails on the two most important areas of performance and cost, so we advise that you give this model a miss.