Microsoft Reserved Partition
A Microsoft Reserved Partition (MSR) is a partition of a data storage device, which is created to reserve a portion of disk space for possible subsequent use by a Windows operating system installed on a separate partition. No meaningful data is stored within the MSR; though from the MSR, chunks may be taken for the creation of new partitions, which themselves may contain data structures.
Formerly, on disks formatted using the master boot record (MBR) partition layout, certain software components used hidden sectors of the disk for data storage purposes. For example, the Logical Disk Manager (LDM), on dynamic disks, stores metadata in a 1 MB area at the end of the disk which is not allocated to any partition.
The UEFI specification does not allow hidden sectors on GPT-formatted disks. Microsoft reserves a chunk of disk space using this MSR partition type, to provide an alternative data storage space for such software components which previously may have used hidden sectors on MBR formatted disks. 
The starting size of an MSR depends on disk size and usually aligns with the following table. This size gets reduced as portions of the MSR are taken to be used, as described above.
|Disk size||MSR size|
|Less than 16 GB||32 MB (32 × 220 bytes)|
|Greater than 16 GB||128 MB|
Beginning in Windows 10, the minimum size of the MSR is 16 MB which the installer allocates by default.
The MSR should be located after the EFI System Partition (ESP) and any OEM service partitions, but it must be located before any primary partitions of bootable Windows operating systems. Microsoft expects an MSR to be present on every GPT disk, and recommends it to be created as the disk is initially partitioned.
- "Windows and GPT FAQ - Windows 10 hardware dev". msdn.microsoft.com. 1 June 2017. Retrieved 12 December 2020.
- "PARTITION_INFORMATION_GPT structure (winioctl.h)". docs.microsoft.com. 5 December 2018. Retrieved 14 December 2020.
- "How Dynamic Disks and Volumes Work". technet.microsoft.com. 8 October 2009. Retrieved 14 December 2020.
- "UEFI/GPT-based hard drive partitions". docs.microsoft.com. 2 May 2017. Retrieved 14 December 2020.
- "Recommended UEFI-Based Disk-Partition Configurations". docs.microsoft.com. 30 October 2012. Retrieved 14 December 2020.
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