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Re: Cannot find core.img on a 3Tb GPT usb disk

From: Chris Murphy
Subject: Re: Cannot find core.img on a 3Tb GPT usb disk
Date: Wed, 14 Aug 2013 11:33:16 -0600

On Aug 14, 2013, at 11:19 AM, address@hidden wrote:

> On Thursday, August 15, 2013 12:22 AM, Chris Murphy wrote:
>> On Aug 14, 2013, at 9:50 AM, address@hidden wrote:
>>> Sector Sizes:     512 bytes logical, 4096 bytes physical
>> This indicates it's a 512e drive, but your original post said it had 4096 
>> byte logical sectors.
> The 512/4096 logical/physical is seen by smartmontools while the 4096/4096 is 
> reported by GNU parted and I guess what the smartmontools reported is more 
> accurate as it reads the drive directly.

Or it's a parted bug. I'd try using a newer version and see if you get 
different results. 3.1 is current.

> I think I just accept cgdisk's default when creating the partition for data 
> and the partition alignment was 256 sectors. Maybe that's the reason why the 
> ef02 BIOS boot partition subsequently created was only allowed to start at 
> the rather large value of 1024KiB.

I'm pretty sure current versions of gdisk/cgdisk default to alignment on 2048 
(logical) sector boundaries. The EF02 partition can be anywhere, grub-install 
will find it. And a start LBA of 2048 for the 1st partition is correct for 512 
native and 512e AF disks.

>>>  Is it then possible to make BIOS recognise this disk to allow booting?
>> It should see it as a 512 byte sector drive, so yes if it really has 512 
>> byte logical sectors.
> Forgive me for these naive question: will changing the sector alignment to a 
> smaller value (using gdisk) make it BIOS bootable? Will such change destroy 
> the data on disk? That is, of course, assuming that the 512/4096 
> logical/physical information is correct.

Alignment doesn't affect bootability, except possibly the drives floating 
around that have a jumper setting for alignment meant for XP. If you're not 
using Windows XP you don't want to have that jumper set. Alignment is about 
performance, to keep the firmware from having to do RWM when file system 4KB 
blocks don't exactly match up with 4KB physical sectors. The easiest way to 
deal with alignment is to start the first partition at LBA 2048, and specify 
partition sizes either in whole GB's or MB's.

If you repartition, yes it destroys data on the disk unless you do a 
move/resize with e.g. gparted.

> Yes, I've successfully booted usb devices so BIOS support for usb booting is 
> definitely there. I'll try to find out whether the USB GPT combination is 
> causing the problem from the vendor. It's not that easy, though. Maybe other 
> reader on this list have some experience to share?

gdisk can convert an MBR partitioned disk to GPT. So if you have a USB flash 
drive you can install a minimal linux OS on, it should use MBR partition scheme 
by default, confirm that it boots, then convert its MBR to GPT and see if it 
still boots.

Chris Murphy

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