Strange booting problem

[Rick Stevens]

On 06/26/2015 04:42 PM, jd1008 wrote:
>
>
> On 06/26/2015 05:29 PM, Rick Stevens wrote:
>> On 06/26/2015 04:05 PM, jd1008 wrote:
>>>
>>>
>>> On 06/26/2015 04:55 PM, Gordon Messmer wrote:
>>>> On 06/26/2015 02:51 PM, jd1008 wrote:
>>>>> Just wondering about the bytes in the first sector which
>>>>> you thought might be boot code that is confusing BIOS
>>>>> to think that my usb drive is bootable.
>>>>> The bytes you already saw are obviously not boot code.
>>>>
>>>> What is obvious to you is not obvious to the CPU, which simply
>>>> executes instructions.  Everything in bytes is 0-446 is boot code,
>>>> whether it does anything useful or not.
>>> Fine! No argument there.
>>> Where do device (or partition) labels reside? In the partitions?
>>
>> fdisk- (dos-) style partition tables do not have partition labels. GPT
>> partitions do. They are 72 bytes long, starting at offset 56 in the
>> partition's entry in the partition table.
>>
>> The location of the partition table is given in an 8-byte value
>> starting at offset 72 in the GPT header. Generally, they start at the
>> second LBA (LBA1) on the disk and are 128 bytes long.
>>
>> Filesystem labels (regardless of DPT or GPT partitioning) are located
>> in the filesystem's superblock(s). They are 16 bytes long starting at
>> offset 120 in each copy of the superblock.
>
> OK. So if only GPT partitions have labels,
> what does mlabel do (i.e. where does it place the label?).
>
> $ yum provides /usr/bin/mlabel
> Loaded plugins: langpacks, refresh-packagekit
> mtools-4.0.18-4.fc20.x86_64 : Programs for accessing MS-DOS disks
> without mounting the disks
> Repo        : fedora
> Matched from:
> Filename    : /usr/bin/mlabel

The location of a filesystem label (if supported) is dependent on the
filesystem type, so perhaps I misled you a tad. Sorry! The 16-byte
label area starting at offset 120 of the superblock I mentioned above
is for ext2|3|4 filesystems.

For FAT12 and FAT16 filesystems, the label is stored in an 11-byte area
starting at offset 43 in the partition's header. For FAT32 filesystems,
it's stored in an 11-byte area starting at offset 71 in the partition's
header.

You really can google this stuff yourself, you know.
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- Rick Stevens, Systems Engineer, AllDigital    ricks at alldigital.com -
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