What is the 128MB partition that is created during the installation of Windows?

What is the 128MB partition that is created during the installation of Windows?

Users may inquire about the 128 MB partition that is created along with the 100 MB partition during the installation of Windows. This is known as a Microsoft Reserved Partition (MSR).

The partition is created during the Windows install.

Microsoft Reserved Partition

An MSR partition is only created under the following conditions:

• When a drive is formatted in a Globally Unique Identifier (GUID) Partition Table (GPT) format
• When the BIOS is set for Unified Extensible Firmware Interface (UEFI) 
• Drives less than 16 GB have a 32 MB MSR, where drives larger than 16 GB have a 128 MB MSR

This partition is required for GUID and must reside before the primary (Windows) partition. Some users may want 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.

Understanding Disk Partitions from TechNet 

Windows and GPT FAQ