Dualboot on separate physical drives (BIOS mode, MBR) - Win10(May) and Ubuntu 20.04

[oldfred]

BIOS systems boot from MBR.
Windows uses boot flag to find primary NTFS partition with boot flag & has more code in partition boot sector.
Grub has code (core.img) in sectors just after MBR (if MBR partitioned or in bios_grub if gpt) that searches for rest of grub in your install. Grub does not use boot flag, but a few BIOS require a boot flag, so we usually suggest you have one partition with boot flag on every drive.

Pictures here worth 1000+ words Older Windows but same for all BIOS versions. See graphic for quick overview of BIOS boot process.
http://www.multibooters.co.uk/multiboot.html
http://www.multibooters.com/guides/v...-sequence.html

[pbear42]

I am no longer getting the initial error, but it's still not booting automatically.

Sorry, but this has gotten too complicated, so I'm going to bow out. Good luck.

[ghristov]

Oldfred,

Going through the latest links you posted took me some time to read and digest. I can understand the boot sequence a bit better now and I can see how unforgiving the Windows boot process can be on some occasions. I eventually managed to configure the two OSes to work together by:

• Creating a 1GiB partition (/dev/sda1) in the beginning of /dev/sda(the SSD placed in the SATA HDD)
• Using the Ubuntu installation media i formatted /dev/sda1 as ext4, marked as /boot and installed the bootloader in /dev/sda

​
This is most probably the same thing that happened last time I successfully dual-booted these two, while installing(probably) the bootfiles in the Windows dir using boot-repair.

Thank you for the help!