The extent to which iPod owners can upgrade their players' internal workings was the subject of debate long before Apple's third-generation hardware prompted buyers to wonder how they could get holder of batteries with longer lifespans. Many folk who, like me, own the original 5GB model have pondered swapping out the hard drive for a more capacious model.

Such upgrades are part of the very design philosophy of the Neuros. Instead of building the battery, hard drive and playback electronics into a single unit, maker Digital Innovations has separated the latter from the others. The battery and HDD sit in a box called the Backpack onto which the Player clips.

The idea is that you've got a system that allows you to, say, pull the Player off a Backpack filled with rock music and clip it onto a second HDD unit containing your classical music collection. In principle, you buy one Player and Backpack, then expand your storage capacity at will by acquiring new Backpacks.

It's a nice idea, but one that's spoiled in the implementation. While the Neuros Player is a light, TV remote control-sized unit, the Backpack - I tested the 20GB model - is a chunky, heavy unit that looks more like a notebook PC's AC adaptor than the sort of thing you'd want to carry around in your pocket or on your belt.

So while the 20GB iPod weighs 158.8g (5.6oz), the Neuros HD weighs 68 per cent more, 266.5g (9.4oz). The iPod's svelte 10.3 x 6 x 1.5cm (4.1 x 2.4 x 0.6in) compares to the Neuros' 13.3 x 7.8 x 3.3cm (5.3 x 3.1 x 1.3in). The Neuros isn't that much bigger, but it certainly feels a lot chunkier.

The base of the Backpack is home for the unit's USB 2.0 port, power connector and the catch used to release the Player. The latter's base provides a USB 1.1 port, plus 3.5mm stereo earphone and microphone jack sockets.

Removing the Player - a process that involves holding down the Backpack catch with one thumb and pushing out the Player with the other - exposes its 50-pin connector. Rough handling could easily bend or damage any of the two rows of 25 pins, so the Player's clearly not designed to be kept for extended periods off the Backpack. In any case, the Player doesn't operate on its own.

That demotes it to control unit. Below the 2in, 128 x 128 LCD, there's a four-way navigation button and Skip Back, Play/Pause and Skip Forward controls. Along one side are five radio station pre-set buttons; on the other side is the 'HiSi' song identification activator button and a control lock switch.

You can buy a separate Player with 64MB, 128MB or 256MB of on-board RAM, but you'll also need a battery-only Backpack. These cost $ 60, $ 70, $ 100 and $ 30, respectively.

Read the full rather lengthy review by The Register here.