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View and Change the Disk Signature in the MBR

Purpose and location of the Disk Signature in the
Master Boot Record and how to see and manually change it.

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.)

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

Fig: 1
regedit screenshot

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.

The above regedit screenshot shows a particularly tidy and short list of items in the MountedDevices key, but this list can get very long with many old and defunct entries, especially after cloning and moving Windows. You may see lines starting PPosDevices\ that have duplicate drive letters but these should be ignored as only the \DosDevices\ lines are current and relevant. (The defunct entries may help in retrieving a previous or lost SIgnature) - info gif Retrieving a lost Disk Signature

View a Signature Before Windows Boots.
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!

editscreen thumbnailPwizard
bootscreen thumbnailGparted
fig:2Window's Boot Menufig:3Edit Boot Options

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.

halt icon Please note that this Edit Boot Options screen is to allow developers and Windows experts to alter boot options before booting, so don't delete or type any text on this screen unless you know what you are doing.

View and alter the signature directly in the Master Boot Record.
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.

Fig: 4
disksignature in mbr

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.

Windows Master Boot Record
The MBR shown above is a Win-7 one, the green section is the boot code and yellow is the partition table. The boot code part is commonly known as the IPL (Initial Program Loader) which gets run by the BIOS and in turn passes control to Partition Boot Records and/or a bootmanager. The Vista/Seven IPL has grown in size slightly from previous Windows versions and now takes up all the space before the error messages. The extra code is to do with the Bitlocker encryption boot routines.

The partition table takes up 4 lines with each line being the table for one partition, which is the reason why you can only have 4 primary partitions on a hard drive. Only 4 primaries is an historical limitation of MBR configured hard drives, but this is now changing with the introduction of the new GPT method of configuring hard drives, where it is possible to have 128 primary partitions. If you were to look at the first sector of a GPT configured hard drive in HxD it would be empty apart from one dummy partition entry for legacy reasons and perhaps a disk signature, which would not have the same relevance at boot time as it does for MBR styled hard drives.

There are actually very few occasions you should be manually changing disk signatures and almost never on a hard drive you are currently booted into. Make sure you understand what you are doing before tampering with signatures or messing with a program as powerful as HxD. One false move and you may never see your data or operating system again. Please read our Disclaimer and Advice pages.

Getting to a Signature from Boot Media.
If you want to access the disk signature on a machine you can no longer boot into and don't want to be removing hard drives then it can be done from a boot disk such as our highly recommend PartedMagic, which is actually a light Linux operating system and toolkit that will boot you into a familiar desktop. Easy to download and make and very user friendly for Windows users it will run from CD/DVD or USB/SD flash devices. We think it is a multibooters must have tool for partitioning and cloning and several other things, so if you have yet to acquired it you now have another reason why we think doing so is time well spent.

While PartedMagic does not yet have a built-in disk sector editor it is easy to add one yourself. The free GPL licenced WxHexEditor is a standalone portable app, so there is no convoluted or difficult Linux app install required. You could just download it directly from the Firefox browser in PartedMagic and run it from the desktop, or preferably have it on a usb stick and run it from there. You can download the Linux version from a Windows machine and extract/unzip the tar.bz2 file with a suitable compression tool such as 7zip or Izarc and transfer the extracted folder to your USB stick. (You can of course use any Live Linux distro on disk/usb instead of PartedMagic but you may have to know your way around Linux admin or file permissions and disk access restrictions).info iconSee this PartedMagic forum post on WxHexEdit.

The best of our tested disk sector editing tools - Windows Linux and Mac.

bullet dotHxD - Hex&Disk Editor - A great free Windows tool with installable and portable versions.
bullet dotWinHex and HexProbe - Fully featured Windows pay-for hex editors that can access disk sectors.
bullet dotHex Editor Neo - Heavily featured Windows Hex Editor and disk editor with both pay and free versions.
The disk edit in the free version expires after 14 days, but standard version is cheap.
bullet dotiBored - Basic but free disk sector editor that has versions for Windows, Linux and Mac
bullet dotWxHexEditor - GPL/free Hex and disk sector editor with versions for Windows, Linux and Mac
bullet dotActive@ Boot Disk - A very easy to download and make bootable Live CD/ USB that includes its
own disk sector hex editing tool. - Basic and simple, but quite pricey.

Spanner iconTech Bench Some other options and info for the more advanced tweakers.
If you already have a bootable Windows Live CD/DVD such as BartPE, WindowsUBCD or Winbuilder, or you are able and motivated to build one, then you can run HxD or any disk editor of your choice from there. Either include it in the build or just as with our above PartedMagic example get a standalone portable version that you can simply transfer to desktop or run directly from disk or usb. There are both installable and portable Windows versions of HxD - download page.

Those familiar with the command-line can view and edit a disk signature with the Windows-7 (not Vista) Diskpart utility. Commands list disk then select disk(x) then detail disk will show you the Disk ID: in the reversed format as explained above. Note that the Disk ID: of a GPT styled hard drive will be a much longer GUID number. To edit the disk signature use the new Uniqueid command. To restore our example signature from above would be uniqueid disk id=55898bc8 - Diskpart can also be run from WinRE on bootable media.

MBRWizard is an excellent free third-party command-line tool that among other things can show and alter the Disk Signature. We see there is now a pay-for GUI version for Windows that also comes with a download to make "bootable Media". We have yet to try it.
Ultimate boot CD - A free bootable CD/USB package of Dos tools that includes a couple of
disk sector editing utilities - DiskSpy and PTS Disk Editor.
The Windows 7 MBR - For the serious stuff about MBR's and so much more visit the Starman's site.
For the pedantic - yes the disk signature has been used before in the boot process.
info gif Windows May Use Signature Syntax in the Boot.ini File




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spacermultibooters.com: Aug 2012 - - reviewed or updated: Mar 2013

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