Windows – Why can’t windows boot from fat32 formatted USB on BIOS computer


I have tried to search for this question and found no answer. So, from what I understand is, the boot process works as following:

  1. POST
  2. BIOS
  3. BIOS calls MBR of selected bootable disk
  4. MBR calls Volume boot code, which is the boot manager
  5. Boot manger loads the kernel and so on…

So if we follow the previous logic, a fat32 partition should be able to boot by the usual bootsect /nt60 .... command, as the disk has MBR, the volume has the boot manager code. However, boot still fails for some reason.(on fat32).
The answer provided by This Microsoft help article states that:

This issue occurs because the USB flash drive is listed as removable media. Therefore, the Windows operating system does not create a master boot record (MBR) on the USB flash drive when you format the flash drive to use the FAT32 file system. The USB flash drive is treated as a super floppy disk. The FAT32 startup code does not support starting a computer from a super floppy disk without an MBR. 

The BIOS tries to transfer the control of the startup from the USB flash drive to the FAT32 startup code, even though the FAT32 startup code does not support this scenario.

However I fail to understand how can MBR be absent???

Lastly, if MBR IS absent, is there some way to add it so fat32 boot is possible?

Best Answer

When running Windows from a FAT32 filesystem there is no filesystem based security (you need NTFS for that).
As a consequence of that such a Windows install would be an open invitation to any virus/malware on the planet.

So running Windows from Fat32 is only useful for installation media. And Microsoft never bothered putting an option in to the standard Windows utilities to write a MBR to FAT32 removable media.

But it is easy to do it yourself using 3rd party USB stick format utilities.