It’s a 10GB unit that serves as a removable external hard disk (USB 1.1) but its primary function, for me anyway, is as an MP3 player. It normally sells for $149 on Amazon but they’ve got a $50 Amazon mail-in rebate which brings it down to $99 and at that price it’s not a bad deal. Powered by 4 AA NiMH batteries, it can play for up to 10 hours on a single charge and there’s an Open Source software project that’s been pretty active for quite a while now called Rockbox.
The way it works is you simply plug it in to your PCs USB port and it appears as a removable drive (just like a USB thumbdrive thingy) and you can copy files in and out at well. Later, in your car, for instance, when you want to play something, you use the controls built on to the player and treat it like you would any Walkman or stand-alone CD player. If you wanna store regular files on it, fine … obviously they won’t play but you can do it if you like.
There are larger players but with the $50 mail-in rebate, I just couldn’t beat it.