Symptoms
What is the 128MB partition that is created during the installation of Windows?
System users may inquire about the 128MB partition that is created along with the 100MB system partition during the installation of Windows. This is known as a Microsoft Reserved Partition, or MSR.
Cause
Partition created during Windows install
Resolution
Microsoft Reserved Partition
A Microsoft Reserved Partition is only created when a drive formatted in a Globally Unique IDentifier or GUID partition table (GPT) format and when the BIOS is set for Unified Extensible Firmware Interface (UEFI). Drives less than 16 GB will have a 32 MB MSR, where drives larger than 16 GB will have a 128 MB MSR.
NOTE: GPT is required for partitions larger than 2 terabytes.
This partition is required for GUID and must reside before the primary (Windows) partition. Some users may wish to delete this partition, however it is required for GPT partition drives. This partition can be viewed in the DISKPART tool, but it is not visible in Windows Disk Management.
More information:
Understanding Disk Partitions from TechNet
Windows and GPT FAQ
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