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Understanding Disk IDs

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Understanding Disk IDs

To enable an operating system to uniquely identify disks, a signature is generated as part of the initialisation 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


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 online at the same time. If you attempt to bring a second disk online, 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. Therefore, Reflect never sets the disk signature to that of the source.


Target disk Signature
 Any restore or clone to a GPT disk
Not changed, or if target not initialised a new one is generated.
All target partitions overwritten
From source disk
Source disk present after restore (e.g. in the case of a clone)
Some original target partitions remain after restore Not changed
Target partitioning scheme different from source (e.g. source MBR, target GPT) Not changed, or if target not initialised a new one is generated.
 Target uninitialised, MBR source disk, target < 2TB  From source disk/image

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Last Modified:14 Mar 2013

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Type: Info

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