After a long time away from Linux I thought: "I'll just add extra SSD, it should be easy..."
Wrong. I messed up everything.

After install, boot-repair tells me:

GPT detected. Please create a BIOS-Boot partition (>1MB, unformatted filesystem, bios_grub flag). This can be performed via tools such as Gparted. Then try again. Alternatively, you can retry after activating the [Separate /boot/efi partition:] option.

How can I add this BIOS-Boot partition without breaking the Ubuntu install?

My system is like:

sda             223,6G                               
└─sda1 vfat     223,6G /mnt/boot-sav/sda1            240

sdb             931,5G                               
├─sdb1 ntfs       223G /mnt/boot-sav/sdb1            
├─sdb2 ntfs       467M /mnt/boot-sav/sdb2            
└─sdb3             16M                               

sdc             489,1G                               
├─sdc1 vfat       512M /boot/efi                     
└─sdc2 ext4     488,6G /                             
sr0              1024M 

sda is empty SSD.
sdb in Windows 10 hard disk.
sdc is current Ubuntu SSD.

Where should I start?

  • You already have that partition as sdc1, I believe. Just boot-repair or your BIOS/UEFI now wants to boot from sda as your first drive, instead of sdc which contains the boot loader and EFI files. Can you just try and swap them around in your UEFI setup? If that doesn't work, you might try swapping their cables. – Byte Commander May 20 '18 at 22:31
  • Not quite. The bios_grub is for BIOS boot on gpt partitioned drives. Normally gpt is used with UEFI and with UEFI boot you must have an ESP - efi system partition which is FAT32 with boot flag. You must be consistent in how you boot installer, and then your hard drive after install. And any repair flash drives. Once using UEFI with gpt always boot in UEFI boot mode. And if Windows boots in UEFI boot mode only use gpt & UEFI boot. But if booting in BIOS mode from gpt, then you must have a bios_grub partition. Best to use same boot mode as Windows and better to use UEFI with newer UEFI. – oldfred May 20 '18 at 22:54

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