The Windows NT operating system from before NT4 till Win-8 will remember the drive letter that has previously been allocated to a partition or drive and always reassign that same letter every time the operating system is booted. This differs from the Win9x generation of operating systems where drive letters were assigned on every bootup in a default order as the partitions and drives were discovered, which meant that a change to partitions or the adding or removal of drives would cause drive letters to change. With NT operating systems the hard drive partition drive letters are maintained by linking them to a partition by using a unique number that is assigned to each partition and stored in the Windows registry. On a classic MBR styled hard drive this unique partition number is generated from the disk signature of the drive and the sector number that a partition starts on, (It's different on GPT drives - see box below). During Windows bootup the stored partition number is compared with the disk signature and if they match then the partition will retain the drive letter that was previously assigned to it.

If a disk signature is changed the partition numbers in the registry become invalid. They no longer match the disk signature of a drive 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 updated to match the new disk signature, then be reassigned a drive letter. These reassigned letters may not match the originals and so drive letters may change. (You will also be informed that 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. It is one of the vagaries of a Window's configured dual or multi-boot setup that in certain circumstances a Windows install may be assigned something other than the usual C: for its drive letter. If you have a Windows operating system 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:. This will mean that as Windows continues to boot and starts looking for files such as E:/Documents and Settings or E:\Users\Name the paths will of course now be invalid and files won't be found. Windows 2K and XP cannot recover from such a drive letter change, but Vista and 7 are a bit more adaptable and can often struggle to a limited desktop, from where it may be possible to manually edit the registry to restore the original drive letter. How to Recover From a Changed Windows Drive Letter.

If you are familiar with the Windows System and Boot partitions and how 2K and XP assigns drive letters then you will know that when you have separate system and boot partitions in a dual or multi-boot setup then the System partition would normally be C: and additional operating systems will be on a Boot partition that could be anything other than C: or the D of the optical drive. It means that when a disk signature problem triggers drive letters to be reassigned in the default order of System partition as C: and Boot partition as D: it will be unlikely that an OS on a Boot partition will get its original letter back.

There was a small change made to drive letter allocation procedures in Windows Vista so that when an install is carried out from bootable media and not from inside a booted Windows operating system the install routines ignore any current letter assignments to existing partitions and allow each added OS to take C: as their drive letter. Every Vista in a dual/multi-boot machine whether on a system or boot partition would be able to see itself as the C: drive, which is a definite improvement, but it does nothing to stop a boot partition operating system being assigned a wrong drive letter after a disk signature change. If anything it makes matters worse because with the decline of optical drives there would have been more occasions where an operating system on a boot partition would have seen itself as the D: drive, thereby making a re-assigning of default letters correct.

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Up to and including Windows Vista a separate boot partition with its inherent capacity for drive letter problems was only something that multibooters were likely to encounter. From Windows-7 however it became the preferred and default configuration to have separate system and boot partitions, even if there is only one operating system on the computer. The system partition is now called the System Reserved partition and this will house the bootmanager files of the Windows install, with the operating system itself being on the separate Boot partition. Hence the old way of re-assigning drive letters in the order of system partition as C: and the boot partition as D: would always be wrong and clearly needed to change.

Reverse order.
From Windows-7 the default letter allocation order for system and boot partitions has been reversed. Now the Boot partition gets priority and will be assigned C: while the system partition comes second and gets D: - that is if it gets a letter. If the System Reserved partition is small and houses only a few boot files then Windows will deem that it holds nothing the average user needs access to and so will not assign a drive letter, thereby removing it from being visible in Windows.

So now every Windows install that sees itself as the C: drive, whether on a separate boot partition or on a combined system/boot partition, will survive a disk signature change and a drive letter reassigning event. A truly welcome improvement and all we need now is for Microsoft to make it that all Windows installs can only ever see themselves as the C: drive. Has this already started to happen?...... we have just noticed that our full retail OEM versions of Windows-8 will not allow the type of install (the Custom install from inside a booted Windows) that would produce a different drive letter. We'll update on this as we get our hands on more final retail versions of Win-8 and do more testing. Have you seen the same? feedback.

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As you can see in the following image the 24 digit unique partition numbers are retained in the registry key of HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices.