How do I figure out if I have an MBR partition type?

stf92 · 04-14-2018, 01:55 PM

Hi: I made

Code:

# dd if=/dev/mmcblk0p1 of=pepe bs=512 count=1

where mmcblk0p1 is the linux partition (only one), and look at this:

Code:

[root@revenge ~]# hexdump -C pepe
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000200

Does it mean I have a GPT partition type. At disk offset 0 I expected to find the MBR. Acording to parted I have a msdos partition table.

coralfang · 04-14-2018, 02:13 PM

Most likely. If i run that on my system (i'm using LILO/dos partition for MBR) i get this;

Code:

00000000  fa eb 21 01 b4 01 4c 49  4c 4f 18 02 b4 44 d2 5a  |..!...LILO...D.Z|
00000010  00 00 00 00 fc f3 a4 50  2a 46 04 00 81 00 80 60  |.......P*F.....`|
00000020  d7 3e f4 2f b8 c0 07 8e  d0 bc 00 08 fb 52 53 06  |.>./.........RS.|
...
...
etc

I believe you can use fdisk to check:

Code:

$ sudo fdisk -l /dev/sda | grep Disklabel

If it's a GPT partition type, you should see

Code:

Disklabel type: gpt

or dos/MBR will be:

Code:

Disklabel type: dos

michaelk · 04-14-2018, 05:23 PM

The MBR signature is the last two bytes. 0x55AA.

rknichols · 04-14-2018, 05:32 PM

Quote:

Originally Posted by michaelk

The MBR signature is the last two bytes. 0x55AA.

But a GPT disk will also typically have a "protective" MBR to keep the disk from appearing empty to non-GPT-aware tools. That protective MBR will also have the magic "0x55 0xAA" in the last two bytes.

If you just run gdisk on the drive, it will immediately tell you what type of partition tables it finds there.

stf92 · 04-14-2018, 05:50 PM

Parted definitely says it's a dos partition table. But how then is the first sector filled with zeroes?

rknichols · 04-14-2018, 07:38 PM

Quote:

Originally Posted by stf92

Parted definitely says it's a dos partition table. But how then is the first sector filled with zeroes?

Only the last 66 bytes contain the partition table and signature. The first 446 bytes can be anything, and are needed only for a primary boot loader.

michaelk · 04-14-2018, 07:50 PM

Quote:

Originally Posted by rknichols

But a GPT disk will also typically have a "protective" MBR to keep the disk from appearing empty to non-GPT-aware tools. That protective MBR will also have the magic "0x55 0xAA" in the last two bytes.

If you just run gdisk on the drive, it will immediately tell you what type of partition tables it finds there.

Your correct. It is the EE partition in the protective MBR that identifies it as a GPT I believe.

stf92 · 04-15-2018, 10:58 AM

Quote:

Originally Posted by rknichols

Only the last 66 bytes contain the partition table and signature. The first 446 bytes can be anything, and are needed only for a primary boot loader.

Allright. But there is nothing at all (all zeroes) in the full 512 bytes!

rknichols · 04-15-2018, 11:49 AM

Quote:

Originally Posted by stf92

Allright. But there is nothing at all (all zeroes) in the full 512 bytes!

Then there is no DOS/MBR partition table present, regardless of what any tool might claim.

michaelk · 04-15-2018, 12:06 PM

Doh... I was tired and couldn't see the forest for the trees and you even posted partition 1.

/dev/mmcblk0p1 is partition 1 and the first 512 should be empty.

/dev/mmcblk0 is the entire device where the MBR lives.

Try

Code:

dd if=/dev/mmcblk0 of=mbr.img bs=512 count=1

rknichols · 04-15-2018, 12:40 PM

And, I overlooked that too. You don't typically expect to find a partition table within a primary partition.

stf92 · 04-15-2018, 03:55 PM

Quote:

Originally Posted by michaelk

Doh... I was tired and couldn't see the forest for the trees and you even posted partition 1.

/dev/mmcblk0p1 is partition 1 and the first 512 should be empty.

/dev/mmcblk0 is the entire device where the MBR lives.

Try

Code:

dd if=/dev/mmcblk0 of=mbr.img bs=512 count=1

My mistake. Now I get my two partitions into the partition table. By the way:

Code:

00000180  47 52 55 42 20 00 47 65  6f 6d 00 48 61 72 64 20  |GRUB .Geom.Hard |
00000190  44 69 73 6b 00 52 65 61  64 00 20 45 72 72 6f 72  |Disk.Read. Error|
0000

Does this mean Grub made the MBR?

michaelk · 04-15-2018, 04:08 PM

Yes, whatever tool used to partition the drive created the partition table and grub installed the boot loader.