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To enable an operating system to uniquely identify disks, a signature is generated as part of the initialization process. In the case of an MBR partitioned disk, this is a 4 byte value, typically presented as an 8 character hex string. This is extended to a 16 byte value for gpt disks, presented as a GUID.

You can see the disk signature by opening a command window, and typing

diskpart

followed by

list disk
sel disk <n>

replacing <n> with the number of the disk you wish to query and then type

detail disk

MBR disk example

DISKPART> detail disk
...
Disk ID: 4936F9A1

GPT disk example

DISKPART> detail disk

...

Disk ID: {C0F4976C-CFA6-45C1-BBCE-3B40016336E5}

MBR partitioned disks

For MBR partitioned disks, the disk signature is relevant for (Vista and later) booting and also to ensure drive letters assignments persist between reboots. As it is used to uniquely identify disks, Windows will not allow multiple disks with the same disk signature to be on-line at the same time. If you attempt to bring a second disk on-line, windows will re-calculate the disk signature on the second disk. This will potentially make it not bootable and also change any drive assignments.

For these reasons, as part of a clone or restore process, Macrium Reflect will recalculate the signature according to the table below. If the signature is recalculated, the boot configuration data (BCD) store is located and updated and the drive mappings in HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices\ are also updated if necessary.

GPT partitioned disks

Unlike MBR partitions, GPT partition entries have an associated unique id. This allows GPT partitions to be uniquely identified without the disk signature making the disk signature redundant for the purposes discussed above. 

Summary

The following table indicates how disk signatures are treated for restore or clone operations: