Microsoft Reserved Partition
A Microsoft Reserved Partition, or MSR, is a partition of a data storage device, which is created simply to reserve a chunk of disk space for possible subsequent use by the operating system software of a Windows operating system (Installed to 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.
|Disk Size||MSR Size|
|Small (Less than 16 GB)||32 MB (32 × 220 bytes)|
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. However, the MSR partition is not actually required for Windows to work, so can be deleted; though doing this can possibly break the boot-loader.
The MSR partition is not visible within the Microsoft Windows Disk Management control utility, but it is listed with the Microsoft Diskpart command line utility.
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