Paul Nicholls Stuff

Simple so far, physical disks can be broken up into partitions, partitions appear as logical disks within the Operating System. The process of making a partition available to the operating system, and assigned it a logical drive is known as mounting. Where this becomes more complicated is that modern OS's allow us to start doing more clever things with our disks and drives... Firstly, we don't need to mount a partition as a logical drive, it can be mounted as a folder on another drive. For example, rather than make my "Cache" drive the G: drive, I can instead make it a folder on my D: drive, like D:\Cache\, or D:\CacheDrive\, or D:\Other Disks\Cache\.

Up until now we've been dealing with "Basic" disks, Windows XP (sorry, not Home Edition!) also lets us have "Dynamic" disks. Now, dynamic disks are a lot more flexible than basic ones. If you're going to do any kind of RAID at software level, you'll need to be using dynamic disks. There's nothing physically different about a dynamic disk, Windows just manages it in a different way.

You can use the Disk Management snap in to make a basic disk dynamic, just right click on the disk and choose the option. Once its partitioned you'll then be able to establish a RAID array of your choice - at software level. What this means is that the operating system itself is responsible for maintaining all the disks in the array, so if using a two disk mirror, the software will write the changes to each of the two disks (whereas with hardware RAID, it writes it once and the hard drive controller will be in charge of generating the two disk writes).

Another feature of dynamic disks is that you can make a single logical drive (or indeed any mount point) that spans more than one physical disk. For example, I hard two 40GB hard drives, I could use dynamic disk features to merge the two 40GB partitions into one 80GB logical drive.

The possibilities are endless, but be warned, making a disk dynamic is supposed to be a one-way process - and in that process Windows changes the volume on the disk in such a way that other operating systems won't understand what's happened to the disk. You won't be able to, for example, use dual boot facilities on a dynamic disk.

I've dealt mostly with Windows in this post, and for that I apologise. Most of these features can be found in other systems too, but that is outside my field of experience. The feature of mounting volumes on folders, rather than logical drives is used extensively in Linux systems (see also, symlinks).