A Microsoft Reserved Partition (MSR) is a partition of a data storage device, which is created simply to reserve a portion of disk space for possible subsequent use by a Windows operating system installed on a separate partition. It should be noted that 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 GPT label for this partition type is
Formerly, on disks formatted using the older MBR partition layout, certain software components used hidden sectors of the disk for data storage purposes. One example of this is the Logical Disk Manager (LDM), which, should the disk be converted from a basic disk to a dynamic disk, would store metadata in a 1 MB area at the end of the disk which was not allocated to any partition.
GPT formatted disks and the UEFI partition specification do not allow hidden sectors. 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, for example LDM as mentioned above, 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.
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.
Some chunks of the MSR partition may also be used for remapping damaged sectors by software (in the device driver), or used as a temporary mirror for critical operations on the core GPT structure or in emergency for fast writing of the internal disk cache memory in case of power failure (many disks have a fast write-through cache memory generally not exceeding 32 MB, and the MSR created by Windows is generally 128MB by default but may be larger in some systems), and for fast recovery when powering up the system. Contents in the MSR generally have some data signature to identify the content and make sure it is meaningful, but this signature is vendor-specific to each hardware component using it.
For normal operation when the system is booted, the MSR partition is usually no longer needed (except with some device-specific power management systems which may use the MSR also as a fast recoverable "scratch" area when the disk is dynamically powered on and off on demand). With hardware RAID managers, the MSR may be much smaller (1 or 2 MB at start of disk is generally enough for proper operation, depending on total disk size and partition alignment parameters).
Disks with large capacities (over 2 TB) used on servers may have a larger fast cache memory, but this cache may be backed up by a frontal (hidden) SSD, so no space is needed in the MSR area of the physical disk for fast recovery in case of power failure or when using dynamic power management. For PCs preinstalled with a single SSD, the OEM recovery partition is usually at end of disk (this allows this OEM partition to be backed up to an external DVD or USB drive and then deleted from the SSD, for extending the main data partition over it after the OEM backup has been done without having to move all sectors on the SSD; during this operation, the small MSR partition may be temporarily used as a scratch area for recovery in case of unexpected failure of the conversion such as battery/power failure). Finally some security tools may add their own data or software components in the MSR for checking the system integrity at boot time. Some UEFI boot tools may also use the MSR as a temporary scratch area. In summary, the actual usage and content of the MSR is very device-specific and invisible to normal application software or to the Windows API, through which the MSR partition is not exposed as a mountable volume (it contains no standard filesystem).
The MSR partition is not visible within the Microsoft Windows Disk Management control utility, but it is listed (as "Reserved") with the Microsoft Diskpart command line utility. On bootable disks for UEFI systems, the MSR partition is generally the second partition just after the first small partition displayed as "System" (about 100 MB) and containing a FAT32 filesystem (to store the UEFI boot data), before other partitions for the actual main partition Windows (usually mounted on volume C:), or for other OSes (on multiboot systems), or for one or several recovery partitions (usually not mounted but visible in the Microsoft Windows Disk Management control utility). The OEM system recovery partition is usually located either at end of disk (for harddisks with capacity below 1TB), or near the start of disk just after the MSR partition on large disks for UEFI-capable systems (OEM recovery partitions may also be hidden in the Microsoft Windows Disk Management control utility, by using a GPT partition attribute flag: their internal filesystem is generally NTFS or FAT32, but they may be mounted with Diskpart).