A physical disk signature is a unique hexadecimal identification string of characters assigned to a computer storage device. The term "physical disk" refers to a hard drive. A computer or computer-like device can use the physical disk signature to tell different storage devices apart and recognize storage devices that have been attached to the system previously. If a computer has two storage devices with matching physical disk signatures or a storage device with an altered physical disk signature, it may encounter problems.
Storage Device Identification
The eight-hexadecimal character physical disk signature allows a computer to tell all storage devices apart, as the odds of two storage devices having the same physical disk signature are under one in four billion. The computer uses the physical disk signature as a reference to help it locate files stored on a device. The signature is a bit like the city part of a mailing address; it doesn't tell you exactly where something is located, but the signature identifies the general area of the information the computer is looking for. Without the disk signature, the computer gets confused when looking for files because it can't tell storage devices apart. An unassigned physical disk signature has a value of 00000000.
Boot Sector Contents
The physical disk signature is stored in something called the Master Boot Record or Volume Boot Record on the storage device. The MBR or VBR is not accessed in the same way that the operating system reads and writes files; rather, it is controlled by the operating system and contains all the necessary information to operate the hard drive, find file locations and boot to the operating system. If the MBR or VBR is corrupted or deleted, the storage device will lose all important information necessary to operate the drive, including the physical disk signature. However, the operating system can attempt to repair the boot record if it is damaged.
Matching Physical Disk Signatures
Under normal circumstances, it is exceedingly unlikely that two storage devices will have the same exact physical disk signature. However, a cloned version of a storage device may have the same physical disk signature as the origin drive. If the two devices with the matching disk signatures are never used at the same time with the same system there's no problem, but a computer will experience a "disk signature collision" if two devices with matching physical disk signatures are connected at the same time. When experiencing a disk signature collision, the computer will be unable to tell the two storage devices apart and will be unable to read either, even if both drives contain identical content.
Changing Physical Disk Signature
The physical disk signature is not set in stone and can be changed. However, changing the physical disk signature can wipe out the file allocation table and render all data on the device unusable. If the physical disk signature is changed, the information stored on the device may be recoverable through a file recovery program or by restoring the original physical disk signature.
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