Microsoft Reserved Partition

The Microsoft Reserved Partition, or MSR, is a partition on a data storage device that is labelled with Globally Unique Identifier (GUID) equal to E3C9E316-0B5C-4DB8-817D-F92DF00215AE. The containing storage device has to use the new GUID Partition Table (GPT) format, not the traditional master boot record (MBR) partition table format. Microsoft Reserved Partition is used to reserve space ahead of time for other use, and it does not contain any meaningful data itself.

Microsoft requires an MSR on every GPT disk, and recommends it to be created as the disk is initially partitioned. It should be located after the EFI System Partition (ESP) and any OEM service partitions, but—most importantly—the first data partition must follow it. Initial size of MSR is 32 MB on disks smaller than 16 GB, or 128 MB on other disks, although it may later automatically shrink on behalf of other partitions, for example during the conversion from basic disk to dynamic disk.

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