Convert dual boot disk into single OS

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  1. tony359
    Posts : 121
    Windows 10 Pro
       New #1

    Convert dual boot disk into single OS


    Greetings all.

    May I ask for a bit of sanity check please.

    I have a PC running two Windows 10 OS's on two separate partitions. Windows asks me which one I want to run when booting up.

    I now want to remove one OS and recover its space.

    If I understand correctly I need to:

    - open MSCONFIG and remove the OS I do not want from the list of OS's.
    - Remove the unwanted partitions
    - Expand the main OS partition

    My only concern is that I see three "small" partitions on the SSD and I just want to make sure I am not removing anything important.

    I am attaching a screenshot of the available partitions. C: is the partition I want to keep and D: is the one I want to remove. Recovery must be a standard Windows one. What about the 99MB and the 474MB ones?

    I'd appreciate your help on this! Thanks!

    Convert dual boot disk into single OS-partitions.jpg
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  3. Samuria
    Posts : 8,113
    windows 10
       New #2

    Both recovery say they are oem not windows the 99mb is the uefi parition which your windows boots from and cant be removed. If you delete d uou will have to move c to the left and then expand
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  4. tony359
    Posts : 121
    Windows 10 Pro
    Thread Starter
       New #3

    Thanks!

    Mine is not a branded PC, I installed Windows 10 from scratch on this SSD which I bought brand new - why do I have two OEM partitions on it?
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  6. Kari
    Posts : 17,661
    Windows 10 Pro
       New #4

    tony359 said:
    Why do I have two OEM partitions on it?
    Because, as illogical as it sounds, default Windows Setup will put the OEM partition first on disk, where it cannot expand when a feature upgrade requires more space. When more space is required, it creates a new one after the C: drive.

    Screenshot from step 2.9 in this tutorial: DISKPART - How to Partition GPT disk | Tutorials

    Convert dual boot disk into single OS-image.png
    (Click to enlarge.)

    Kari
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  7. tony359
    Posts : 121
    Windows 10 Pro
    Thread Starter
       New #5

    Thanks for your help!

    I am still confused. What is an OEM partition? Google suggests partitions allocated by manufacturer - such as HP or DELL.

    I am also not sure I understand when you say "When more space is required" - can you elaborate further? Does Windows 10 dynamically allocate its partitions?

    Thanks!
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  8. Kari
    Posts : 17,661
    Windows 10 Pro
       New #6

    On a GPT formatted disk, Windows puts the recovery (OEM partition) first on disk, then EFI system partition and a Microsoft Reserved (MSR) small, unformatted partition. Then comes C: where Windows will be installed.

    Recovery partition is required for Windows repair, troubleshooting and such. When you upgrade, it will be upgraded with parts of the new Windows version. Because default Windows Setup put it in a place where it cannot expand, it "steals" some space from C: putting itself on that "stolen" space.

    From here on, always when new upgrade comes, setup can now use this new OEM partition, again stealing space required and expanding itself backwards to this new space.

    OEM partition, as Microsoft calls it now, is a confusing name for it because, as you said, it can also mean the partition for factory reset.

    Kari
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  9. tony359
    Posts : 121
    Windows 10 Pro
    Thread Starter
       New #7

    I see, thanks. Now you're mentioning it, I read something about that. So those OEM partitions are used by Windows updates?

    If I understand correctly, I just need to remove D, move C back and then expand. The last OEM partition will be used by Windows Updates for the next Windows Version (Kind of small though? Will Windows expand that partition when the next version is out?)?
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  10. Kari
    Posts : 17,661
    Windows 10 Pro
       New #8

    tony359 said:
    So those OEM partitions are used by Windows updates?
    Upgrades, not updates.


    tony359 said:
    If I understand correctly, I just need to remove D, move C back and then expand.
    Correct. The thing is, you will need a third party tool to move / expand partition backwards. Windows Disk Management can't do it.

    tony359 said:
    The last OEM partition will be used by Windows Updates Upgrade for the next Windows Version?
    Yes.


    tony359 said:
    Will Windows expand that partition when the next version is out?
    Upgrade does not need to expand OEM partition (old name was WinRE), if there's currently enough space for upgrade recovery tools. When space is required, it shrinks C: and expands to that space.

    Quote from Microsoft Docs:

    Recovery tools partition

    This partition must be at least 300 MB. The Windows Recovery Environment (Windows RE) tools require additional free space:
    • If the partition is less than 500 MB, it must have at least 50 MB of free space.
    • If the partition is 500 MB or larger, it must have at least 320 MB of free space.
    • If the partition is larger than 1 GB, we recommend that it should have at least 1 GB free
    .
    Kari
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  12. tony359
    Posts : 121
    Windows 10 Pro
    Thread Starter
       New #9

    Thanks for clarifying.

    Just wondering if it's worth wiping the disk, reinstall and then restore the C partition using Macrium?
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  13. Ztruker
    Posts : 14,046
    Windows 11 Pro X64 22H2 22621.1848
       New #10

    Try deleting D then moving and expanding C, just to get the experience. If any problems, you can always wipe, install and restore.
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