The Disk Signature is a unique ID number that Windows writes to a specific location inside the Master Boot Record (MBR) on the first sector of every hard drive that it sees. This signature dates back to Windows NT 3.5 in the mid 1990s and was then referred to as the Fault Tolerance Signature because its primary purpose was to help a booting or booted operating system uniquely identify individual hard drives in a fault tolerant dynamic disk raid configuration. It still has this use and others in the IT world of servers and disk clusters but from 2007 and the release of Vista it also became a vital component in how the Windows boot files find the hard drive of the operating system to be started. In the absence of a sought after disk signature the Windows bootmanager will in its default configuration be unable to locate operating systems on classic MBR configured hard drives. (Note that it is different on the new GPT styled hard drives where the MBR disk signature no longer has any involvement in the boot process.)

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Determine a Disk Signature.

From inside Windows perhaps the easiest way to see a disk signature is just to open the Windows registry editor regedit and navigate to the key - HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices

As you can see in the right hand pane of fig:1 each device that Windows has allocated a drive letter will have its own \DosDevices\ line with a long alpha/numeric number in the Data column. The devices with the shorter 24 digit numbers will be devices that are actually partitions on a hard drive - in our example this is the C: E: F: and G: drives. The A: and D: devices have much longer numbers and are not devices that typically use MBRs or their disk signatures - for example Floppy, CD/DVD, USB flash keys/cards, or GPT configured hard drives. The DosDevices data numbers on GPT styled drives will include partition GUID numbers as opposed to MBR disk signatures and will be double the length at 48 digits.Other devices will be more than double this length again.

The C: drive on our machine has a 24 digit number so we know this is a partition on an MBR hard drive and we can see the Disk Signature of that drive in the first 8 digits of its number. The C: drive is therefore a partition on an MBR hard drive with the Disk Signature of c8 8b 89 55 and the G: drive which has the same first 8 digits is another partition on the same hard drive. The E: and F: drives both show a different disk signature of 5a 8a f8 78 and so this tells us that this particular PC has a second MBR hard drive that these other partitions are on.

If you have a non-bootable drive that you can connect to a working PC, even by USB, then as long as it contains a partition that Windows can mount and assign a drive letter to, you will see the disk signature for that drive in the corresponding data number.

There is a way with Vista and Seven that you can be shown the Disk Signature via the Windows bootmenu screen. This however does rely on the bootmanager working properly and being able to identify the correct hard drive before it can look at that drive's disk signature. The bootmanager uses the disk signature to identify hard drives, so this won't work if you are having boot problems due to a changed disk signature!

If you have a dual or multiboot machine using the Window's bootmanager then you will naturally see the boot menu screen at every startup (fig:2). In our example here there is only one boot option so this screen will not normally appear by itself at boot time. To get it to show, tap the spacebar repeatedly at the end of the bios POST (Power-On Self Test) and just before Windows starts to boot, (Win-8 see here). Highlight a boot entry and press F10 and you will go to the Edit Boot Options screen (fig:3) for that operating system, where you will see the disk signature of the hard drive that operating system is on. (On GPT styled drives it will be the longer GUID number of the drive that will be shown.) In our example the signature is shown as 55898bc8 but you need to split it into 4 pairs of 55 89 8b c8 and then reverse the order of the pairs to see it in the same format of c8 8b 89 55 which you see in the other examples on this page.

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You may already have a third party tool such as the bloated Paragon or Acronis partitioning utilities that can show the MBR code and let you see and edit the disk signature. If you don't then we recommend this excellent free app that among other things is a raw disk editor, HxD - Freeware Hex and Disk Editor. Any hard drive whether bootable or not that you can hook-up to a Windows machine, even by USB, will be explorable by HxD.

In the following screenshot we have HxD running from inside Windows and we have opened Disk 1 on our machine and have resized the app window down to display just the first sector (Sector 0) and we have circled the signature and colored some sections for easier identification. Be aware that not all Hex/Disk editors will automatically display the MBR in the format shown here. The MBR sector will always end with what some call the "Magic Number" which is 55 AA and we have shaded in red. Make sure that it is at the bottom of the two right most columns of a 16 column grid and the disk signature will be as we have circled.

The Disk Signature can be any 8 alpha/numeric digits from zero to 9 and from A to F and only has to be unique to that of any other MBR configured hard drive that is going to be visible to Windows. The algorithm that Windows uses to generate these digits does suggest it is not 100% random because it sometimes throws out similar pairs, but from experience we believe this is just an affectation you can ignore. Once you know where the signature is it is fairly straight forward in HxD to open the disk in write-mode and change it. You will need administrator privileges in Windows and will have to click through a few HxD warnings, but you just over-type the current digits and Save All.