Before starting, make sure you have a backup, and make sure to have a linux live boot ready to rescue your system. It's easy to mess this up!
- Use gdisk to convert the partition table to GPT.
gdisk /dev/sda
- Create the "BIOS boot" partition that GRUB needs.
n
to create a new partition. Needs to be about 1MB. You can probably squeeze this in from sectors 34-2047. UseL
orl
to look up the code for "BIOS boot" (ef02). - Write the new partition table.
w
- Reload the partition table.
>
partprobe /dev/sda