About Contribute Disclaimer Contact News
The information on this site is offered in good faith and no responsibility can be accepted for misuse that leads to loss of data or damaged hardware.
Help Us Improve
Have you spotted an error, or found something confusing or ambiguous? Help us improve our articles......

Google Advert Links
tech insight guyTech Insight
Explaining it in plain language.
spacer Multibooters.com

The Disk Signature and Windows Drive Letters

How drive letters are maintained and how
they are linked to a hard drive's Disk Signature.

My Computer Drive Icons

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).

Default Drive Letters.
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. info IconHow to Recover From a Changed Windows Drive Letter.

Other Problems After a Disk Signature Change.
On a natively configured Windows machine with an MBR styled boot hard drive a disk signature change will of course cause you a more immediate problem than drive letter issues. Whether multiboot or not the new generation of Windows operating systems use the disk signature in the boot process to locate a hard drive, so a changed or missing signature will prevent bootup from even starting. It will only be after you have repaired this little problem that you will then find out if you are in for drive letter grief. info icon Fixing a Signature Problem
boot error screen graphic
Boot Partition.
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.

Windows 7 and 8 System Reserved Partition.

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.


The Unique Partition Numbers.
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.
regedit drive letters screenshot

The highlighted number is that of the C: drive and we can tell from its first 8 digits that the disk signature of the hard drive is e8 b4 12 f4. The E: drive is showing the same disk signature in its partition number so this tells us that E: is a separate partition on the same hard drive that the C: partition is on. The F: drive also has a 24 digit unique partition number, but the disk signature here is 68 5a 12 8d, which tells us that this machine must have at least two hard drives. The A: D: and G: devices and any device with a longer data number are not devices that use MBR disk signatures for identification, such as floppy, CD/DVD, USB sticks, or GPT styled hard drives.

GPT Styled Hard Drives

The new GPT (GUID Partition Table) style of partitioning that will be common on hard drives in Windows-8 supplied machines from Oct 2012 will no longer use any MBR disk signature as part of the unique partition number in the Windows registry. Instead the 48 digit long unchanging partition GUID numbers of the GPT system will be used, which means that the disk signature will no longer have any involvement in maintaining drive letters on a GPT styled hard drive. info icon Is your Drive MBR or GPT?

The new GUID partition table has been designed to work with the new EFI firmware that is set to replace the traditional BIOS firmware on computer motherboards. A GPT styled hard drive has a special hidden partition called the EFI System Partition (ESP), which essentially contains much of what the MBR does on tradition hard drives. On a GPT drive Windows will put its bootmanager files on this partition, which means that Windows will always have separate System and Boot partitions, hence the necessity for the change to drive letter allocation order, and why the System Reserved partition was introduced on MBR styled hard drives. info icon An Introduction to UEFI and GPT


multibooters logo multibooters.com
creative commons licence

All product and company logos, icons and images that are reproduced on this site are the property and trademarks of their respective owners and are used here merely to illustrate their products, NOT to indicate or infer any endorsement or partnership unless otherwise stated. We have endeavored not to reproduce copyrighted images or graphics or infringe upon the rights of any trademark or copyright holder. If you believe or suspect we may have breached the permissible use of copyrighted material then please bring it to our attention. The reproducing or copying of original material from this site is currently allowed for non commercial purposes with the provision that your source is clearly indicated and that a back-link to the referenced information is included. Thank You. See our full  Terms of Use.  Material on this site is not guaranteed to be free of errors.   Multibooters.com   2012 - 2013

Home Sitemap Privacy-Policy About Contact/Feedback Donate Copyright/Legal