What Is a Disk Signature?

Disk signatures 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 ever recorded on an individual computer since Windows was installed is stored in the HKEY_LOCAL_MACHINE hive in the Windows Registry, at HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices.

If you are 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 above 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

Multibooters.com has more information on how to read the hexadecimal disk signature values in the Windows registry, including which values pertain to the partitions that make up a hard drive.

Disk Signature Collisions and Why They Happen

Disk signature collision in Windows are rare, but they do happen when two storage devices have the exact same disk signature.

Probably 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 they're identical copies.

Identifying a Disk Signature Error in Windows

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

Windows also won'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 will be taken offline and will not be 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

Fixing 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, is as easy as turning the hard drive back online from within Disk Management, allowing a new disk signature to be created.

If the hard drive that has the disk signature collision error is used for booting to run 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 at Multibooters.com and 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, but this 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 (possibly without losing all of the drive's data), see the tutorial at HowToHaven.com.