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Can BIOS limitation prevent Windows from booting if no boot flag set?

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I know from experience that I can boot Windows on my machine even if its partition doesn't have its boot flag set by using a boot loader like Grub, and this is also the experience of most of my friends. But I have one friend who claims he must have the boot flag set on his Windows partition or he can't boot it, even with a boot loader like Grub that doesn't require having the boot flag set to boot a partition. He claims it is a limitation of his BIOS; I don't understand the logic behind that, because I thought that once BIOS hands the boot process over to the master boot record (MBR), then BIOS doesn't care whether any partition has its boot flag set, only the MBR might. For non-configurable boot loaders like the Win XP MBR, the Win MBR relies on one of the partitions having the boot flag set, because that is how the Win XP MBR decides which partition to boot. But that is not the case with Grub, as Grub can happily boot a partition regardless of whether it is the active partition (i.e. has its boot flag set on). Or at least that has been my experience.

So does anyone think that BIOS could somehow be stopping Grub from booting his Windows partition just because it is not the active partition? Because that is not the experience of myself or most of my friends. Thanks in advance for any help.
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Neil Jones

Neil Jones

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As a general rule the active partition is the one that gets loaded first unless the desire is overridden in some fashion. This is effectively the closest one gets to having boot flags and its an area that has nothing to do with the BIOS.
You quite often find that custom BIOSes such as those made for HP, Compaq, Packard Bell, etc - anything with a built-in recovery partition - have their own ways of forcing the system to load from somewhere else other than the default active partition.

In days of old, Windows 95, 98 and ME required you to have an active partition (and for one or the other to be the only OS on the machine - officially anyway) otherwise none of them would load. These days its possible to set a system up to boot from any partition you care to mention regardless of whether its active, primary, logical or anything providing you provide a way to get to them from the active partition in a Windows environment.

Grub, being a bootloader, is irrelevant of the BIOS. The BIOS finishes what it does then tries to load an operating system off anything it can find. Once its done that that's the end of it so I don't see how a BIOS limitation can even be involved as by this point the BIOS is finished.
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