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.
nto create a new partition. Needs to be about 1MB. You can probably squeeze this in from sectors 34-2047. UseLorlto look up the code for "BIOS boot" (ef02). - Write the new partition table.
w - Reload the partition table.
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partprobe /dev/sda