Windows 10 Clean Install: MBR or GPT?

SPARTAN VI

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For my own sanity going forward, I was wondering what heuristics are involved in Windows 10 automatic selection when creating a new partition from unallocated space during the Win10 installation?

I recently did a clean install of Win10 x64 on my Crucial P1 NVMe, and I was not given a prompt to choose my partitioning scheme for the drive. Win10 automatically chose MBR and completed the install. So I started over, formatted the drive, and used diskpart to force GPT partitioning on my P1 instead and completed the installation as expected.

Later found that I had USB booting set to "legacy mode" in my UEFI. Is this the reason Win10 chose MBR for my installation?
 

Spartacus09

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Yes, if your bios was set to ahci aka legacy, thats why it defaulted to mbr.

Windows install itself can be burned (or usb key iso’ed) as either uefi(gpt non-csm) or as legacy as well (mbr+gpt-csm).
 

ir0nw0lf

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I get annoyed at older Windows 7 and even some Windows 8 boxes I work on that still have legacy mode turned on / UEFI features off. If I reload them with Windows 10 I have to spend the time combing through the friggin' BIOS to make sure all that is set correctly.
 

BinarySynapse

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If the system boots in legacy BIOS mode (whatever the vendor chooses to call that), then you will get MBR partitioning. If it boots in UEFI mode (meaning Windows installer was loaded from an .efi module), then you'll get GPT formatted partitions.

Yes, if your bios was set to ahci aka legacy, thats why it defaulted to mbr.

Windows install itself can be burned (or usb key iso’ed) as either uefi(gpt non-csm) or as legacy as well (mbr+gpt-csm).

AHCI has nothing to do with this.
 

SPARTAN VI

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Found some Microsoft docs that spell it out a little clearer, but the cliff-notes version of it is:

  1. If you boot into UEFI mode, then the GPT device will self-identify and Windows 10 will build the partition using the GPT partitioning scheme.
  2. If you boot into legacy mode aka BIOS mode, then Windows 10 will automatically select the MBR partitioning scheme instead.
    • Even in the above scenario, you can still manually wipe then setup a GPT partition using diskpart commands. However, you will still need to boot into UEFI mode to proceed with the Win10 installation. Win10 installer will not allow you to proceed with the installation of a GPT partition while booted in legacy mode.
Does that sound about right to everyone?
 

SvenBent

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i woudl do GPT when you have the option. but i would not worry enough to change a current drive on mbr
 

SPARTAN VI

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Right, but you cannot utilize the quick boot features on MBR, which basically translates to a 20-30 second boot versus ~5 seconds I get today.

My question was about spelling out Windows 10's heuristics because it does not give you an option when it automatically creates the partition.
 

SvenBent

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Right, but you cannot utilize the quick boot features on MBR, which basically translates to a 20-30 second boot versus ~5 seconds I get today.

My question was about spelling out Windows 10's heuristics because it does not give you an option when it automatically creates the partition.

Thats a very valid point but S3 sleep resolves that for me

and yes i believe you are correct on the eufi/gpt vs bios MBR
 
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DeaconFrost

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For future reference, if you use diskpart to configure the primary drive, you have total control. SHIFT+F10 during the first installer screen brings up a command prompt. I'll select the primary drive, then run a convert gpt to be sure. Now, when Windows 10's installer continues, I'm guaranteed to have a GPT partition.
 
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