On a UEFI/GPT disk computer Microsoft requires a EFI system partition formatted FAT32 and a Microsoft System Reserved Partition that is 128MB on any disk larger than 16GB. See What is the MSR here. Edit: As hollowface posted MSR is 16MB in Windows 10.
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.
What partitions are required by Windows?
For UEFI systems, the boot drive must contain an ESP, an MSR, and at least one basic data partition that contains the operating system. Only one ESP should exist on a system even if multiple operating systems are installed on that system. In a mirrored boot configuration there may actually be two drives with an ESP but they are considered to be a redundant copy of the same ESP. Each data drive must contain at least an MSR and one basic data partition.
Hard to believe but I have seen up to 7 partitions on an upgraded Windows 8 to 8.1 or 10 computer, a total freaking mess.
Edited by JohnC_21, 14 September 2015 - 04:59 PM.