What Is a Disk Signature and Why Is It Necessary?

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 it 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

You can display the disk signature in Command Prompt, with the diskpart command.

1. Open Command Prompt and enter diskpart.

2. When that command completes, enter list disk.

3. Enter select disk 0 (but choose the number that corresponds to the disk you're interested in).

4. Finally, enter uniqueid disk to display the disk signature for the disk you just chose.

Disk Signature Collisions and Why They Happen

Disk signature collisions in Windows happen when two storage devices have the same 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 signatures.

Windows also doesn't accept two identical disk signatures in Windows 11, 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

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 signature.

If the hard drive that has the collision error is used to boot Windows, then fixing it may be a bit more difficult. Visit Microsoft's website for steps to fix a disk signature collision error and screenshot examples of the errors you might encounter in Disk Management.

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 the drive's data), see HowToHaven.