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 Windows Hard Drive Disk Signature

How the latest Windows operating systems
use the Disk Signature in their boot process.

mbr signature stamp
Windows will stamp a unique ID number into the first sector of every MBR hard drive it uses. This need not be a concern if you intend using only Microsoft’s own bootmanager and you also follow their recommended procedures for installing Windows. If however you stray from the Microsoft path there are a few ways that this disk ID number could catch you out.
Much of what this Disk Signature has been used for since its inception in the mid-90s is not relevant to our cause here but if you want to know more and even go on to see and edit disk signatures then start with this page.

Booting Windows.
There comes a point during the computer boot sequence when control is passed to a Windows component on the boot hard drive. From Windows Vista onwards this component is called bootmgr and this is the Microsoft bootmanager that has taken the place of the previous ntldr bootmanager that was part of XP and earlier WinNT operating systems. Just like the ntldr before it this new bootmanager is capable of booting not only the WIndows install it is a part of, but also other Windows operating systems that you may have on separate partitions or on additional hard drives in the PC. Unlike the ntldr however the bootmgr uses the disk signature to locate the hard drive that it wants. When bootmgr is tasked with starting an operating system it will scan all connected hard drives looking for the disk signature of the drive that the target OS is on. If the sought after signature is not found then bootmgr cannot continue with its task and so will stall with an error message. Even if you only have one hard drive and one operating system the disk signature check is still a default part of the boot process, hence issues with the disk signature can affect not only multibooters, but any Windows machine, (for GPT styled hard drives - see side box below).

Find Windows graphic

Finding winload.exe

The ultimate goal of bootmgr is to find and start the bootloader of its target operating system. The bootloader (or osloader) is the small program that begins the bootup of an operating system, which in new-generation Windows is a file called winload.exe, that is located in the Windows/System32 folder. There could be various reasons why bootmgr may fail to find or start winload.exe and when that happens the error message that will be generated may not be very helpful in telling you the exact cause of your problem. In Windows Vista the error message will tell you that winload.exe could not be started and that it may be 'Missing or Corrupt' which can be misleading if the issue is a disk signature one. For Windows-7 the error message was changed to 'The boot selection failed because a required device is inaccessible' which may be slightly more helpful in indicating a disk signature issue, but less helpful if the problem was indeed a missing or corrupt winload.exe file.

How to Lose a Disk Signature.

There are various things you might do that could change or remove a Disk Signature. It resides on the same first sector of a hard drive as the MBR (Master Boot Record) and so replacing or repairing the MBR can alter or overwrite an existing signature. Many operating systems will write a new MBR to a drive as part of their install procedure, though most up-to-date OSes will now retain any existing disk signature they find, but older ones such as all Win9x versions will zero out the part of the sector where the signature resides. Most modern partitioning and hard drive tools will also now maintain signatures, but older tools such as fdisk will remove the signature if a command like fdisk/mbr is used. Many third-party bootmanagers will change or replace parts of the MBR but again most up-to-date ones will know to leave signatures alone, however older ones such as the still popular XOSL will overwrite a signature during its install.

Moving Windows graphic

Moving Windows to a Different Hard Drive.

The cloning, imaging or deploying of the latest Windows operating systems to a different hard drive is going to run into problems come boot time if the signature on the new drive is not the one that bootmgr will be looking for. To avoid this problem a full sector-by-sector clone of the entire hard drive could be employed to move Windows and the signature and everything else in one unit. Or a signature could be manually changed on the new drive to match the one on the parent hard drive. Or the bootmgr's data store (the BCD) could be updated so that a different disk signature is associated with the OS in question. If you do end up duplicating an existing disk signature be aware that this could cause problems as Windows won’t tolerate seeing two hard drives with the same signature. If you boot one drive while the other is connected the signature on the non-system drive may be automatically changed, which will mean that all Windows installs on that drive will then fail to boot. A Vista or earlier Windows operating system will auto change a duplicate signature, where as Windows-7 and later will isolate the drive and report a Signature Collision.

The other crucial factor when moving new Windows is the starting sector of its partition. The bootmgr program uses sector numbers to locate partitions and so if the start of a partition has moved on a drive then the number of the first sector will have changed and bootmgr will fail to find that partition and consequently winload.exe, so the familiar error message will appear. For a much fuller examination of the new Windows boot files and how they operate.... see info iconThe New Generation Boot FIles - bootmgr and BCD.

GPT Styled Hard Drives
There is a new way of partitioning a hard drive that is going to become the norm over the next few years. The GUID Partition Table (GPT) is set to replace the traditional MBR style of partitioning. From Oct 2012 many new Win-8 installed machines will come with GPT styled drives and the associated UEFI bios firmware that is required to boot from these drives.

Operating systems have to be GPT aware to use such drives and the Windows OSes that can will no longer use MBR signatures to identify them but rather a unique GUID number that each drive will already have as part of the GPT partitioning scheme. These GUID numbers will be considered and even called the drive's disk signature and will be used in the boot process in the same way MBR signatures are used on traditionally partitioned drives. info icon MBR or GPT
Fixing a Signature Problem.
If Window's own bootmanager is being used exclusively and a machine has been arranged to a fairly simple Microsoft default configuration then the automatic boot repair feature of the Windows Recovery Environment (WinRE) is pretty good at identifying a disk signature problem and adapting the bootmgr's data store to accept a new signature. If however you have a non standard setup or you are using a third-party bootmanager then the WinRE auto repair may make matters worse for you by trying and failing to convert everything to Microsoft norms. Consult our Guide to WinRE for more on this. If you are familiar with the DOS command line then there are a couple of Microsoft text based tools that could be used, or you could try one of the third-party point and click apps that can alter Windows boot settings. More details on manually fixing a Disk Signature problem can be found on this page.

Avoiding Signature Problems.
It is possible to circumvent the disk signature dependence of bootmgr but there are qualifying conditions to using this approach. It will only work for Windows installs that have all their own boot files on their own partition, so it can't be employed if you have a separate System Partition for the main boot files, or if the Windows bootmanager is being used to boot your additional operating systems. If however you are using a third-party bootmanager and all your Windows installs are independent, then you can configure bootmgr to ignore the signature and always just start the operating system that is on the same partition as itself. The big advantage of this is that it allows cloning or moving Windows to other drives and partitions without experiencing any disk signature issues. See...info icon Generalize the Boot Configuration Data store.



Spanner guy iconTech Bench Extra tech stuff for the enthusiasts and the curious.
The IT-pro's deployment tool Sysprep will among other things prepare a Windows install to expect a few changes the next time it is booted. The generalize command will remove bootmgr's disk signature dependence for the very first reboot, so allowing it to find and boot Windows, then adapt itself to its new surroundings and to a changed or missing disk signature. This is only reliable for boot drive primary partitions and for simple and native Microsoft configurations. With Windows-7 it became the default configuration to have separate System and Boot partitions and so the sysprep tool was updated to prepare bootmgr on the system partition to find Windows on a different boot partition. Again this is only reliable for basic and native Microsoft configurations.
info iconSysprep Technical Reference info gifNew BCD Element


up arrowTop of page....
spacermultibooters.com: July 2012 - - Reviewed or updated: Mar 2014

Google's New Desktop Strategy: Build it straight into Windows by way of Chrome itself. www.infoworld.com

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