I own multiple PC and on some of them, I am able to boot to Windows installer using a USB flash drive formatted in NTFS (same one for all the PC), but not for others.

PC that I have tried this on:

  • (able to boot from NTFS) Desktop with motherboard ASUS PRIME Z390-A and a 9th-Gen Intel Core processor

  • (not able to boot from NTFS) Surface Pro (5/1796) with i7-7660U (a 7th-Gen) and corresponding motherboard

  • (able to boot from NTFS) ASUS ROG Zephyrus M (GM501) with Intel HM370 Chipset and i7-8750H (an 8th-Gen processor)

All of them have CSM disabled (or simply unavailable) and the USB has a GUID Partition Table (GPT) and only a single primary partition.

From the information I gathered online, UEFI-based PC will only boot from an EFI System Partition or a FAT32 partition. But the evidence from my experience clearly tells me otherwise.

Why is that?

  • UEFI motherboards will boot WinPE/WinRE from an NTFS formatted partition on Windows 10, however I can't provide what happens on the backend within the Windows bootloader - that information is accessible on Microsoft Docs within their Manufacturer related docs. It used to be that Win10 required a FAT32 formatted drive to boot WinPE/WinRE on UEFI motherboards, however than changed ~2017ish to NTFS. (I don't know why the change occurred, but this should also be able to be found on Microsoft Docs or TechNet.) – JW0914 May 16 '20 at 12:08
  • @JW0914 I see. So it's a change in the specification from Microsoft's end. And I guess the timing makes sense too (since ~2017 is about the time where most of the PC's are UEFI-based by default (as opposed to BIOS or UEFI-with-CSM by default).) – dragonxi May 16 '20 at 22:19

UEFI uses file system drivers to access boot volumes - and the only required driver is FAT (12, 16 and 32). This allows for more drivers to be loaded from a FAT partition, working its way up to boot an OS system partition, that is formatted in a different file system.

Of course nothing stops a vendor from integrating more drivers directly into his UEFI implementation - as obviously some of you examples are doing.

  • AFAIK, something in the Windows bootloader changed ~2017ish to allow booting WinPE/WinRE from NTFS on UEFI motherboards... I'm not sure what changed, just that before the change, a USB drive had to be formatted as FAT32 in Rufus (using a Win10 installer ISO) to EFI boot WinPE/WinRE, and after the change, NTFS was required and FAT32 wouldn't boot. – JW0914 May 16 '20 at 12:13
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    @JW0914 The change you mention is the install image breaking through the 4GB barrier, but it is unrelated: The OQ is about what happens before - how does control flow ever reach a bootloader on an NTFS partition. – Eugen Rieck May 16 '20 at 13:46
  • I have zero understanding of the technicalities of bootloaders, but in regards to the first part, I'm not aware of any Windows install.esd/install.wim exceeding 4GB in size... Microsoft has always been quite intentional about keeping them ~3.8GB in size. The ISO itself can be >4GB, provided no individual file is, as the ISO itself isn't being copied, it's content is. – JW0914 May 16 '20 at 13:52
  • I am at home now, but I am quite positive to have had a 4.2G install.wim last time. I'll take a look on Monday. – Eugen Rieck May 16 '20 at 16:24
  • I see, so theoretically a motherboard vendor (like ASUS and MSI) can integrate drivers for many more formats (such as Linux's ext4, Windows Server's ReFS, etc.) into UEFI firmware right? So that explains a lot... – dragonxi May 16 '20 at 22:12

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