[syslinux] USB boot problems on Gigabyte GA-M55Plus-S3G

[Pete Batard]

On 2014.01.15 12:45, Thomas Schmitt wrote:
>> I have a
>> strong suspicion that this LBA "flag" is a fake flag that simply indicates
>> if a FAT partition type was set as CHS (0x06, 0x0b for FAT16 and FAT32
>> respectively [1]) or LBA (0x0e, 0x0c for FAT16 LBA and FAT32 LBA
>> respectively [1]).
>
> This sounds plausible.

I have now confirmed that this is indeed the case.

Booted GParted on a bootable UFD that had a single FAT32 LBA partition 
(type 0x0c @ address 0x1c2 of the MBR), and confirmed that GParted 
listed the flags: Boot, LBA.

I then cleared the LBA flag, and checked the MBR, to find out that the 
partition type had now been switched to 0x0b (FAT32 CHS).

> (I still wonder how the C/H and H/S factors are determined
>   for the C/H/S case.)

Not sure either.

> Can you advise Ronald a way to get rid of the active/bootable flag ?
> Just to see how much stake this quirk of his board has in the
> overall problem.

Easiest way to do this from Windows is to use HxD [1], which is what I 
used to check my MBR.

In HxD select Extras -> Open Disk and pick the relevant one under 
"Physical disks" (make sure to use Physical disks, not Logical disks, as 
you won't have access to the MBR otherwise).

For an USB Flash Drive, the disk will probably be listed as "Removable 
Disk 1". Also make sure to _uncheck_ the "Open as Readonly" option, 
since we'll want to change content.

 From there, you can hex edit your MBR as you see fit, or any other 
sector of your disk for that matter, so, if you want to remove the 
bootable flag, just change the 0x80 at address 0x1be to 0x00.

Finally, HxD can also be very convenient when someone on this 
mailing-lists asks you to provide a dump of an MBR from Windows ;)
Simply select the content from the first sector, and go to File -> 
Export -> Editor View. You can then save an hexdump like the one 
attached (which shows the FAT32 partition @0x1c2 now being set to 0x0B 
instead of 0x0C, after removing the flag in GParted), and provide it to 
a mailing list.

Regards,

/Pete

[1] http://mh-nexus.de/en/hxd/

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