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Spock45
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PostPosted: Fri Jan 02, 2015 8:07 pm    Post subject: Bootloader Not Detected Reply with quote

I've tried installing Gentoo several times, sometimes using the Gentoo handbook, sometimes using DasGregor's Gentoo tutorials. Each time, the install seems to go without incident, but when I reboot my PC, it doesn't detect the bootloader. (I've used both GRUB Legacy and GRUB2.) I think the problem may be that I'm not putting a filesystem on my bios_boot partition (not to be confused with the /boot partition,) but being a Gentoo newbie, I am in no way sure if I'm right -- it's just a wild guess. If I'm right, please say so. If not, then say what you think the problem is. Thanks :) !
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NeddySeagoon
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PostPosted: Fri Jan 02, 2015 8:30 pm    Post subject: Reply with quote

Spock45,

Your bios_boot partiiton is only required on (U)EFI systems.

If you use a GPT disklabel on a BIOS system, it may check the protectve MSDOS partition for the bootable flag then not go any further.
BIOS systems will not check for flags in GPT partition - they cannot read the GPT partition table.

Proceede as follows, here is my /dev/sda as an example.
Code:
# fdisk -t dos /dev/sda

Welcome to fdisk (util-linux 2.25.2).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

The size of this disk is 2.7 TiB (3000592982016 bytes). DOS partition table format can not be used on drives for volumes larger than 2199023255040 bytes for 512-byte sectors. Use GUID partition table format (GPT).

Command (m for help): p

Disk /dev/sda: 2.7 TiB, 3000592982016 bytes, 5860533168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0x00000000

Device     Boot Start        End    Sectors Size Id Type
/dev/sda1  *        1 4294967295 4294967295   2T ee GPT

Partition 2 does not start on physical sector boundary.


Notice the Boot flag set. as indicated by the *

Do not use fdisk to create/delete partitions while its in -t dos mode.

-- edit --

No filesystem is needed on your bios_boot partition. grub2 will write it directly, if it needs to.
Any filesystem there will be ignored.
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Computer users fall into two groups:-
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Spock45
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PostPosted: Sat Jan 03, 2015 2:59 am    Post subject: Reply with quote

NeddySeagoon wrote:
Spock45,

Your bios_boot partiiton is only required on (U)EFI systems.

If you use a GPT disklabel on a BIOS system, it may check the protectve MSDOS partition for the bootable flag then not go any further.
BIOS systems will not check for flags in GPT partition - they cannot read the GPT partition table.

Proceede as follows, here is my /dev/sda as an example.
Code:
# fdisk -t dos /dev/sda

Welcome to fdisk (util-linux 2.25.2).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

The size of this disk is 2.7 TiB (3000592982016 bytes). DOS partition table format can not be used on drives for volumes larger than 2199023255040 bytes for 512-byte sectors. Use GUID partition table format (GPT).

Command (m for help): p

Disk /dev/sda: 2.7 TiB, 3000592982016 bytes, 5860533168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0x00000000

Device     Boot Start        End    Sectors Size Id Type
/dev/sda1  *        1 4294967295 4294967295   2T ee GPT

Partition 2 does not start on physical sector boundary.


Notice the Boot flag set. as indicated by the *

Do not use fdisk to create/delete partitions while its in -t dos mode.

-- edit --

No filesystem is needed on your bios_boot partition. grub2 will write it directly, if it needs to.
Any filesystem there will be ignored.


So I should just leave an MBR partition table (my computer does use BIOS, not UEFI.)
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NeddySeagoon
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PostPosted: Sat Jan 03, 2015 11:13 am    Post subject: Reply with quote

Spock45,

You can use either GTP or MSDOS partition tables.
When you make a GPT partition table, you get a free MSDOS partition table too.
When your BIOS boots, as its BIOS, not UEFI, it looks at the MSDOS partition table for the bootable flag, which is not set, then it will not load the boot loader.

As you already have GPT, set the bootable flag on the MSDOS partition and the BIOS will be happy. It will then load the boot loader.
Grub legacy will work on this hybrid arrangement but grub2 will be more robust.
Both grubs want to 'embed' some code. grub legacy can't do that as your GPT partition table is using the space it wants, so it uses a fallback mechanism.
grub2 will use your bios_boot partition for this.

Both work but grub2 is more robust in this situation.

The way ahead is
Code:
fdisk -t dos /dev/...
and set the bootable flag.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.
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