The Icy Dock MB982SPR-2S (or to give it its full name: the Icy Dock MB982SPR-2S Full Metal Dual 2.5in to 3.5in SATA HDD & SDD Converter with RAID for PC & Mac Pro – which must surely be a record) is an intriguing device for Mac Pro owners.

Essentially it is a 3.5in metal caddy that fits into a Mac Pro hard drive slot, and inside is space for two 2.5in hard drives (the type normally used in laptops). It features a built in hardware RAID system enabling you to combine the two hard drives into one single drive that is recognised by the Mac Pro.

Although it’s possible to use any kind of 2.5in drive, we imagine that its primary use will be to add twin SSD (Solid State Drives) units to a Mac Pro. Most SSD drives are sold in the smaller 2.5in format, and are designed to fit in laptops not desktops.

But SSD drives offer a massive performance boost so are as much of interest to Mac Pro owners as they are to laptop owners. Indeed, Apple started adding them to its soon-to-be-extinct Xserve systems as boot drives alongside the regular hard drive, and SSDs are sold as an option on many Macs.

The Icy Dock MB982SPR-2S  is also interesting because it enables you to double the number of hard drives you can add (up to eight in total in the four bays).  And because it has hardware RAID you don’t have to rely on the software solution provided in Mac OS X.

The key advantage, though, is that you can stripe together two SSD drives to provide even faster performance than a single SSD drive would provide (which would be in itself far faster than a regular spindle drive).

Of course, if you’re going to place two SSD drives into a computer you need to think about cost. Although SSDs have dropped in price dramatically, they still start at around £100 for a 64GB drive and go up to a whopping £1,000 for a 512GB (with a 256GB drive in the middle around £450).

Typically the idea is to place the Mac OS X boot installation and applications on the SSD drive and then all your documents (video, photographs, iTunes folder and so on) on a regular hard drive. The SSD provides the speed; the regular hard drive the space.

If you take this approach you can use the Icy Dock to combine two small 64GB drives to provide 128GB of space for the Mac OS X boot drive, and use your regular hard drive as a second drive for storage.

We tested the Icy Dock with two Kingston SSDNow V+100 64GB drives (with a SRP of £100 each). We used AJA System Test to monitor performance and cloned Mac OS X using SuperDuper from the regular hard drive (a stock Western Digital included with the Mac Pro) to the new Icy Dock setup.

So how much faster is it? The stock hard drive provided a write speed of 43.4MBps and a read speed of 43.7MBps. The Icy Dock SSD combo provided a read speed of 247.2MBps and a write speed of 243MBps.

That’s a 547 per cent performance gain from two £100 drives and £70 caddy; certainly nothing to be sniffed at. The read speed is actually fairly typical of a single SSD inserted directly into the Mac Pro, but the write speed is roughly double that of a solo SSD drive.