Windows 2000, XP and 2003 provide a feature called Dynamic Disks. A dynamic disk can contain simple volumes, spanned volumes, striped volumes, mirrored volumes, and RAID-5 volumes. When using dynamic storage, you can perform disk and volume management without the need to restart the computer. Once the change has been made to Dynamic Disks, they cannot be changed back to Basic Disks without deleting the partitions and rebuilding the disk. This can be troublesome, thankfully there is a way to hack it back to a Basic Disk. Caution: Make sure you have a full, verified backup of all your data before you proceed. If you make an error during this procedure the disk will become corrupt and data loss can occur. Read more at Rods Blog.