Understanding Disk IDs

- Overview

- Viewing Disk IDs

- MBR Initialized Disks

- GPT Initialized Disks

- How Does Macrium Reflect Handle Disk IDs?

Overview

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.

Viewing Disk IDs

Disk IDs can be viewed using the Diskpart utility that is included in Windows. To launch Diskpart, first open an elevated command prompt, then run the following command:

diskpart

The local disks can then be displayed using the following command:

list disk

Next, select the disk whose IDs you want to view:

select disk 1

Replace 1 with the relevant disk number you want to query.

Finally, enter the following command:

detail disk

This command will output lots of different information about the disk. The 'Disk ID' section will display the disk ID and will look similar to the example below:

MBR Disk:

Disk ID: 4936F9A1

GPT Disk:

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

MBR Initialized Disks

For MBR partitioned disks, the disk ID is relevant for (Vista and later) booting and also to ensure drive letter assignments persist between reboots. As it is used to uniquely identify disks, Windows will not allow multiple disks with the same disk ID to be online at the same time. If you attempt to bring a second disk online with the same disk ID, Windows will recalculate the disk ID 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 Initialized 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. 

How Does Macrium Reflect Handle Disk IDs?

Windows does not allow multiple disks to be online with the same MBR Disk ID or GPT GUID. For this reason, Macrium Reflect will always ensure there are no duplicates when restoring or cloning. The following table indicates how disk signatures are treated for restore or clone operations: