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Re: grub-mkrescue: Problem with MBR partition table at start of EFI part

From: Thomas Schmitt
Subject: Re: grub-mkrescue: Problem with MBR partition table at start of EFI partition
Date: Sat, 11 May 2019 14:05:25 +0200


> The Windows 8.1 ISO image that I downloaded from Microsoft yesterday
> also cannot be booted from a USB drive on neither this Macbook or
> another computer.

I wonder how other people can get to the impression that their attempts
succeeded. (Will have to wait for somebody who is willing to go on with
experiments after perceived success.)

> I will continue testing the Windows 10 USB install medium:
> /dev/sdb1  *     2048 15728639 15726592  7.5G  c W95 FAT32 (LBA)

That's not an EFI System Partition.
So i assume the USB stick boots by BIOS and x86 machine code in the MBR.
Unless we believe in EFI firmware that hops on arbitrary partitions ...

> So I calculate 512*2048+454=1049030.
> address@hidden ~$ sudo dd if=/dev/zero bs=1 count=16 seek=1049030
> of=/dev/sdb
> It still boots.  I believe what I calculated was wrong.

You zeroized the LBA field but also the block count field.

It should have been count=4 for the most minimal surgery (LBA=0x00000000).
With count=16 you would have to seek to 512*2048+446 and afterwards would
have to write a new partition table entry with LBA=0, non-zero block count,
and probably a non-zero partition type.

>  I do not continue.

Then let's disinfect our brains and go on with some useful work. :))

Have a nice day :)


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