1

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?

2
  • 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
3

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.

8
  • 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
  • 1
    @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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.