Vista's MBR Disk Signature

http://www.multibooters.co.uk/mbr.html

The Disk Signature and Drive Letters

Windows NT operating systems will remember the drive letters that have previously been assigned to partitions and drives. This differs from Win9x where drive letters are assigned on every bootup in a default order as the partitions and drives are discovered, which means changes to partitions or the adding of drives can cause letters to change. With NT the current hard drive partition drive letters can be maintained by linking them to a partition by using a unique number that is assigned to each partition and stored in the Windows registry. This unique partition number is generated from the disk signature of the hard drive and the partition offset. On Windows bootup the partition number is compared with the disk signature and if they match then the partition will be assigned the same drive letter that had previously been allocated to it.

If a disk signature is changed then Windows unique partition numbers become invalid. They no longer match the MBR disk signature and therefore drive letter assignments are lost. On first reboot after a signature change each partition on the affected drive will have its unique number recreated to match the new disk signature, then be reassigned a drive letter. These reassigned letters may not match the originals and so your drive letters may change. (You will also be informed that your new device/s have been installed and a reboot is required).

When you are using the Windows built-in bootmanager to dual or multiboot it is vital to maintain drive letters because Windows might not see itself as the default C: drive. If you have a Windows install that considers itself to be say the E: drive and a disk signature change causes this assignment to be lost, then during early bootup Windows will reassign drive letters in the default order, which will be the System partition as C: and the Boot partition as D:, which means later in the boot process the registry paths such as E:/Windows and E:/Documents and Settings, will be invalid. Windows 2K and XP cannot recover from such a drive letter change, but Vista seems to be a bit more adaptable and will struggle to the desktop, from where it is possible to edit the registry to restore the required drive letters, (see Drive Letter Problems). Be aware that with Vista it is possible when using the Microsoft bootmanager to have the Vista install see itself as the C: drive even when it is in fact a Boot partition to a different System partition. In this situation a drive letter re-ordering event caused by a signature change will assign the default D: letter to the Boot partition, causing the aforementioned problems.