What is a Microsoft Reserved Partition (MSR)?
The Microsoft Reserved Partition (MSR) reserves space on each disk drive for subsequent use by operating system software. GPT disks do not allow hidden sectors. Software components that formerly used hidden sectors now allocate portions of the MSR for component-specific partitions. For example, converting a basic disk to a dynamic disk causes the MSR on that disk to be reduced in size and a newly created partition holds the dynamic disk database. The MSR has the Partition GUID:
DEFINE_GUID (PARTITION_MSFT_RESERVED_GUID, 0xE3C9E316L, 0x0B5C, 0x4DB8, 0x81, 0x7D, 0xF9, 0x2D, 0xF0, 0x02, 0x15, 0xAE)
What disks require an MSR?
Every GPT disk must contain an MSR. The order of partitions on the disk should be ESP (if any), OEM (if any) and MSR followed by primary data partition(s). It is particularly important that the MSR be created before other primary data partitions.
Who creates the MSR?
The MSR must be created when disk-partitioning information is first written to the drive. If the manufacturer partitions the disk, the manufacturer must create the MSR at the same time. If Windows partitions the disk during setup, Windows creates the MSR.
Why must the MSR be created when the disk is first partitioned?
After the disk is partitioned, there will be no free space left to create an MSR.
How big is the MSR?
When initially created, the size of the MSR depends on the size of the disk drive:
On drives less than 16GB in size, the MSR is 32MB.
On drives greater than or equal two 16GB, the MSR is 128 MB.
As the MSR is divided into other partitions, it becomes smaller.