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.
The GUID Partition Table (GPT) label for this partition type is
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. Such software components can create a small software-component specific partition from a portion of the space reserved in the MSR partition.
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|
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.
- "Microsoft Reserved Partition". Windows and GPT FAQ.
- "How Dynamic Disks and Volumes Work". Microsoft TechNet.
- "Windows and GPT FAQ - Windows 10 hardware dev". Microsoft Developer Network. Microsoft. Retrieved 2016-11-06.
- "UEFI/GPT-based hard drive partitions".
- "Recommended UEFI-Based Disk-Partition Configurations".