[GLLUG] Booting biosboot

Henrik Morsing henrik at morsing.cc
Thu Mar 17 09:47:31 UTC 2022

On Tue, Mar 15, 2022 at 11:49:10AM +0000, Tim Woodall wrote:
>Do you mean a partition table that looks something like this?
>root at xen17:~# fdisk -l /dev/sda
>Disk /dev/sda: 931.51 GiB, 1000204886016 bytes, 1953525168 sectors
>Disk model: Samsung SSD 870
>Units: sectors of 1 * 512 = 512 bytes
>Sector size (logical/physical): 512 bytes / 512 bytes
>I/O size (minimum/optimal): 512 bytes / 512 bytes
>Disklabel type: gpt
>Disk identifier: DAD8D524-D8F7-D04D-A21E-C8E14F39285B
>Device      Start        End    Sectors   Size Type
>/dev/sda1    2048       4095       2048     1M BIOS boot
>/dev/sda2    4096     264191     260096   127M EFI System
>/dev/sda3  264192 1953525134 1953260943 931.4G Linux filesystem
>Then you don't always need the bios boot partition.

Yes, that's the one. Centos refuses to install without it.

>IIRC you only need a bios boot to use legacy boot. If you're exclusively
>EFI (dirac doesn't support anything else) then the bios boot partition
>won't be used.

It's a very old laptop, BIOS + GTP (on one disk).

>IIUC the problem is that grub needs some disk space for legacy boot and
>the GPT format doesn't have enough unused space for grub to sneek in so
>it needs an explicit partition reserved for it.

It's mystifies me that Ubuntu on the same disk doesn't need this...

Anyway, I realised the reason Centos didn't show up in the boot menu was that it installed GRUB on the other disk to what BIOS boots.

sda is a straight BIOS/MBR disk, sdb, which BIOS boots and has Ubuntu & Centos, is a GPT disk.

Henrik Morsing

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