bjuanico

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. Use L or l to look up the code for "BIOS boot" (ef02).
Write the new partition table.

w
Reload the partition table. > partprobe /dev/sda

	/* 'rm' file deletion utility for GNU.
	Copyright (C) 1988-2017 Free Software Foundation, Inc.

	This program is free software: you can redistribute it and/or modify
	it under the terms of the GNU General Public License as published by
	the Free Software Foundation, either version 3 of the License, or
	(at your option) any later version.

	This program is distributed in the hope that it will be useful,
	but WITHOUT ANY WARRANTY; without even the implied warranty of

	SHA-265-secretPhraseBytes A: 15ffffffe2ffffffb0ffffffd3ffffffc338ffffff91ffffffebffffffb0fffffff1ffffffef60ffffff9effffffc41942c20ffffffe320ffffffceffffff94ffffffc65fffffffbcffffff8c331244ffffff8effffffb225
	SHA-265-secretPhraseBytes B: [21, -30, -80, -45, -61, 56, -111, -21, -80, -15, -17, 96, -98, -60, 25, 66, 12, 32, -29, 32, -50, -108, -58, 95, -68, -116, 51, 18, 68, -114, -78, 37]
	SHA-265-secretPhraseBytes C: â°ÓÃ8?ë°ñï`?ÄB ã Î?Æ_¼?3D?²%
	secretPhraseBytes A: 313233343536373839
	secretPhraseBytes B: [49, 50, 51, 52, 53, 54, 55, 56, 57]
	secretPhraseBytes C: 123456789
	publicKey A: ffffffeffffffffafffffff3ffffffd97b7fffffffe4fffffff1ffffff851a3b165effffffaaffffffda2963ffffffc2ffffffc0363249fffffffc3efffffffc5ffffffe4ffffffaf3ffffffe1ffffff822e
	publicKey B: [-17, -6, -13, -39, 123, 127, -28, -15, -123, 26, 59, 22, 94, -86, -38, 41, 99, -62, -64, 54, 50, 73, -4, 62, 15, -59, -28, -81, 3, -31, -126, 46]
	publicKey C: ïúóÙ{äñ?;^ªÚ)cÂÀ62Iü>Åä¯á?.
	publicKeyHash A: ffffffc410ffffffb064ffffffea39ffffffec7e4ed3