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I booted from my USB flash drive to install Windows 10 with Fall Creators Update. When booting, I had to change the boot mode to legacy in order to see my USB flash drive in the bootable devices list.

While installing Windows, I deleted all partitions on my drive, and let Windows create what it needs, which currently are a 500 MB "System Reserved" partition and another 476 GB primary partition. I noted that in a past clean install, Windows created 3 partitions, the name of the other which I cannot remember.

Now, I am unable to boot into Windows unless I enable legacy mode, otherwise in UEFI mode it shows 'No boot device found'. This applies to both my Asus X751L running American Megatrends BIOS and my Acer Aspire S5 running InsydeH2O BIOS.

How can I boot in UEFI mode?

marked as duplicate by Ramhound, Pimp Juice IT, Run5k, magicandre1981 windows-10 Oct 21 '17 at 18:01

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How can I boot in UEFI mode?

Use the MBR2GPT tool built into Windows 10 1703 (Creators Update) and 1709 (Fall Creators Update). Microsoft has a technical article with a video that might be helpful.

Convert from BIOS to UEFI with MBR2GPT

Once you have converted your installation to GPT, you will have to enable UEFI mode, in order for Windows to actually boot.

  • Thank you. As this wasn't a problem in the last clean installation, why did the BIOS use MBR instead of GPT? Also, next time, how and during which step in the installation process can I convert from MBR to GPT? – George Tian Oct 22 '17 at 7:28
  • You can’t convert during the installation process. As for the reason MBR was used, you booted to the installation media, with UEFI mode disabled more than likely. In order to install Windows on a GPT partition, you must have UEFI mode enabled, and either have a EFI partition or your entire HDD must be unallocated space. – Ramhound Oct 22 '17 at 16:08
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once you have installed in Legacy mode, you cannot revert back to UEFI mode (at least without spending a lot of efforts which is possibly not worth it). because Legacy mode uses MBR records while UEFI mode uses a special partition named EFI (3rd partition) where your UEFI records reside and their corresponding (pointing GUIDs and which folders with bootloaders responsible to load that OS on EFI partition) indexes are written to your BIOS (SPI chip) directly by your OS installer.

UEFI is not quite stable (especially on notebooks) where more than one OS installed, UEFI records can create clutter and lead to bios corruption or boot lockdowns, only way to fix would be reflashing eeprom chip (generally same chip where your BIOS software is) or replacing it (I've personally experienced.) as a result, Legacy (MBR) mode installations are "safer" (maybe biased) over EFI.

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    "once you have installed in Legacy mode, you cannot revert back to UEFI mode" - This is incorrect. MBR2GPT exists to seamless convert MBR to GPT thus allowing this to happen.. – Ramhound Oct 21 '17 at 11:03
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    "UEFI is not quite stable " - What are you talking about. UEFI has been around for almost a decade. It is extremely stable. – Ramhound Oct 21 '17 at 11:06
  • Secure boot mis-lockups, EFI clutters, regarding UEFI clutter when multiple OS'es have been installed. Especially every SPI chip manufacturer its own, not very standard approach on maintaining these records. reddit.com/r/chrultrabook/comments/5aakcy/… forums.lenovo.com/t5/ThinkPad-11e-Windows-13-E-and/… en.wikipedia.org/wiki/… , also refer to superuser.com/a/1261096/785001 – Vishukani Oct 21 '17 at 11:33
  • you can revert disk GPT, but you cannot have EFI partition to be created Disk Partitioning format alone is not enough to convert a MBR bootloader to EFI (requires corresponding entries to be written to BIOS, requires an EFI partition to be built manually, requires a GUID to assign, requires an EFI bootloader to copy to that partition, requires a binary record of loader config to be written manually, requires to MOVE current partition on disk geometry and still not guaranteed to work as still our registry, ntoskrnl.exe, our windows services and device manager expects a MBR loaded system ). – Vishukani Oct 21 '17 at 11:40
  • if MBR2GPT can do it, then we can easily award MBR2GPT to be best software of last decade :) – Vishukani Oct 21 '17 at 11:43

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