MSR Size Curiosity - GPT secondary disks

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  1. TV2's Avatar
    TV2
    Posts : 1,771
    W10 Pro 20H2
       #1

    MSR Size Curiosity - GPT secondary disks


    Recently working on backups on a PC with 5 HDDs in it and noticed that two 640GB GPT disks had 16MB MSR partitions.

    I thought that the size of the MSR was determined by the disk size: less than 16GB = 16MB (Windows 10), and greater than 16GB = 128MB.

    Both of these disks were deleted to all unallocated, and then changed from MBR to GPT disks, then formatted NTFS in Windows Disk Management.

    Shouldn't these MSR be 128MB?

    MSR Size Curiosity - GPT secondary disks-macrium-disks-marked.jpg
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  2. dalchina's Avatar
    Posts : 29,843
    Win 10 Pro (1903)
       #2

    References and even MS reps are a bit coy on this e.g.
    Microsoft Reserved Partition - Wikiwand
    https://answers.microsoft.com/en-us/...6-8c82d828631f
    (See reply by Aaron_Sum)
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  3. Posts : 356
    Windows 10
       #3

    UEFI/GPT-based hard drive partitions

    16 MB is OK for Windows 10.
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  4. dalchina's Avatar
    Posts : 29,843
    Win 10 Pro (1903)
       #4

    That's often what it is, but there are other references- rather the point of interest here e.g.
    https://www.dell.com/support/article/en-uk/sln288535/what-is-the-128mb-partition-that-is-created-during-the-installation-of-windows?lang=en

    https://docs.microsoft.com/en-us/pre...ectedfrom=MSDN
    • A Microsoft® Reserved Partition (MSR)

      We recommend that you add an MSR to each drive for disk management. The MSR is a reserved partition that does not receive a partition ID. It cannot store user data.

      The size of an MSR partition is 128 MB.


    https://docs.microsoft.com/en-us/pre...ectedfrom=MSDN
    How big is the MSR?

    When initially created, the size of the MSR depends on the size of the disk drive:


    • On drives less than 16GB in size, the MSR is 32MB.
    • On drives greater than or equal to 16GB, the MSR is 128 MB.

    As the MSR is divided into other partitions, it becomes smaller.
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  5. Bree's Avatar
    Posts : 18,614
    10 Home x64 (20H2) (10 Pro on 2nd pc)
       #5

    I can't say I've ever seen any of my MSR partitions actually used for anything...

    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 can create a small software-component specific partition from a portion of the space reserved in the MSR partition.
    Microsoft Reserved Partition - Wikipedia

    TV2 said:
    Shouldn't these MSR be 128MB?

    No, not with W10...

    Microsoft said:
    Beginning in Windows 10, the size of the MSR is 16 MB.
    https://docs.microsoft.com/windows-h...ectedfrom=MSDN
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  6. TV2's Avatar
    TV2
    Posts : 1,771
    W10 Pro 20H2
    Thread Starter
       #6

    Yes, I've read many of those articles. The mystery is why some drives are 128MB and others are 16MB on the same PC.

    One clue is that the two 1TB drives were installed and formatted way back in Windows 7, and the two 640GB drives were added recently - Windows 10 1903.

    Maybe recent versions of Windows 10 just use 16MB on data drives and those articles are just old and have not kept up. (like me!)
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  7. Bree's Avatar
    Posts : 18,614
    10 Home x64 (20H2) (10 Pro on 2nd pc)
       #7

    TV2 said:
    Yes, I've read many of those articles. The mystery is why some drives are 128MB and others are 16MB on the same PC...
    ...Maybe recent versions of Windows 10 just use 16MB on data drives and those articles are just old and have not kept up. (like me!)

    Typically a 128MB MSR means that the disk was first formatted under Windows 8/8.1. Windows 10 will use a 16MB MSR for all GPT drives that it formats from scratch.
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  8. Posts : 356
    Windows 10
       #8

    MSR can even be 0 MB. It happens in the disk of this computer, which was converted with MBR2GPT.EXE about 3 years ago. The conversion process didn´t create an MSR partition.
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  9. storageman's Avatar
    Posts : 1,020
    Windows 10 Pro 20H2 19042.572
       #9

    Anibor said:
    MSR can even be 0 MB. It happens in the disk of this computer, which was converted with MBR2GPT.EXE about 3 years ago. The conversion process didn´t create an MSR partition.
    Yes - I noticed that also. But question is what happens when you need to have 15 partitions on that drive ? I always assumed that the reason for the 16mb MSR partition is that the original partition table area is not large enough to handle more than the MBR maximum # of partitions, so I assume a link is placed in the original Partition table pointing at the 16mb MSR partition.
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  10. Megahertz's Avatar
    Posts : 2,381
    Windows 7 HP 64 - Windows 10 Pro - Lubuntu
       #10

    A MSR partition should be only on the Win 10 drive. All other drives (data drives) should have no MSR partition.
    The disks probably once had Win 7 (128M MSR) or Win 10 (16M MSR) on it and only the the Windows partition was cleaned when the correct would be to delete all partitions till one and only one unallocated space then created a new partition (GPT or MBR) and from this partition make sub divisions (other partitions)
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