What Is a Disk Signature and Why Is It Necessary?

Why disk signatures are necessary and how to fix collisions

A hard drive disk.

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A disk signature is a unique, identifying number for a hard disk drive or other data storage device stored as part of the master boot record. An operating system uses disk signatures to differentiate among storage devices on your computer. Other terms for disk signature include disk identity, unique identifier, HDD signature, and fault tolerance signature.

How to Find a Device's Disk Signature

In Windows, a list of every disk signature recorded on an individual computer since Windows was installed is stored in the HKEY_LOCAL_MACHINE path in the Windows Registry, at HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices.

If you're not familiar with the Windows Registry, there are some tutorials available to help.

A disk signature consists of eight alpha-numeric digits from 0 to 9 and A to F. The following is an example of the hexadecimal value of a disk found in the HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices registry location, with the first 4 bytes (8 digits) being the disk signature:

44 4d 49 4f 3a 49 44 3a b8 58 b2 a2 ca 03 b4 4c b5 1d a0 22 53 a7 31 f5

Disk Signature Collisions and Why They Happen

Disk signature collisions in Windows happen when two storage devices have the same disk signature. The most common occurrence is when a drive has been cloned, sector by sector, to make an identical copy, and a user then attempts to mount it alongside the original.

A similar scenario is when backup software or virtualization tools make a virtual hard drive from a physical hard drive. Using the two together at the same time can result in a disk signature collision error because the drives are identical copies.

How to Identify a Disk Signature Error in Windows

In older versions of Windows (such as Windows Vista and Windows XP), the disk signature of a disk reporting a signature collision is changed automatically when it's connected because Windows doesn't allow two disks to function at the same time if they have identical disk signatures.

Windows also doesn't accept two identical disk signatures in Windows 10, Windows 8, and Windows 7. However, in these versions of Windows, the second drive that creates the signature collision is taken offline and isn't mounted for use until the collision is fixed.

A disk signature collision error in these newer versions of Windows might look like one of these messages:

  • This disk is offline because it has a signature collision with another disk that is online.
  • ​This disk is offline because it has a signature collision.
  • The boot selection failed because a required device is inaccessible.

How to Fix a Disk Signature Collision Error in Windows

disk management

To fix a disk signature collision error for a hard drive that only stores data and doesn't have the Windows operating system installed to it, such as a backup drive, turn the hard drive online from within Disk Management. This process creates a new disk signature.

If the hard drive that has the disk signature collision error is used to boot Windows, then fixing the collision may be a bit more difficult. You'll find the steps to fix a disk signature collision error and screenshot examples of the errors you might encounter in Disk Management in TechNet Blogs.

More Information on Disk Signatures

Replacing or repairing the master boot record, installing a new OS, or using a disk partitioning tool can overwrite a disk signature. This process is common only in older systems and tools. Most modern operating systems and partitioning programs keep the existing signature.

For a tutorial on changing a disk signature (without losing all of the drive's data), see the tutorial at HowToHaven.com.