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RE: legacy-bios and uefi boot usb stick

From: John Frankish
Subject: RE: legacy-bios and uefi boot usb stick
Date: Sun, 14 May 2017 13:25:56 +0000

>>>> I've made a couple of dual legacy-bios/uefi boot usb sticks using 
>>>> grub-2.00.
>>>> Both of the usb sticks will uefi boot linux, but there appears to be a 
>>>> problem with legacy-bios boot of linux.
>>>> One of the usb sticks gives an "invalid partition table!" error
>>> What displays the message ? GRUB or the BIOS firmware ? Do you get the GRUB 
>>> rescue prompt ?
>>> Some BIOS firmwares require that a primary partition has the boot flag.
>> It seems as though it is grub displaying the message.
>> In one case I get the "invalid partition table!" message and have to 
>> cycle the power
> If the message was displayed by GRUB, I believe that you would at least get 
> the GRUB rescue prompt.
After videoing the screen, I see that the error message is:

Error: file '/EFI/BOOT/grub/i386-pc/efi_gop.mod' not found
Error: no suitable video modes found

efi_gop* is in i386-efi and x86_64-efi, but not in i386-pc.

If I add "vga=792" then grub gives an error message that this is depreciated, 
but the frame buffer is set to 1024x768. If I try "vga=895" the native 
resolution of 1366x768 is not set even though it is present in "Xvesa 
-listmodes" and is set with uefi boot.

>> It is true that the usb stick that boots has the efi partition set as 
>> bootable and the
>> usb stick that fails to boot does not. Since both boot uefi I assumed this 
>> did not matter.
> The boot flag is meaningless for UEFI boot, since boot sectors are not used.
>> Can fdisk be used to set the partition as bootable without data loss?
> Yes.
Indeed - I set the second partition bootable and now it will boot in 
legacy-bios mode - thanks

>>> This looks like a very old version of fdisk. Cylinders are obsolete. Add -u 
>>> to use sectors as unit instead.
>> It is a recent busybox version of fdisk.
> Too bad is still uses obsolete metrics.
>> By my calculation and by gparted, there is about 1.27mb before the first 
>> partition.
> Whatever. A GRUB BIOS core image takes less than 100 kB.

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