I've moved HD with Ubuntu v18 to HD caddy (DVD slot) and in place of that HD inserted a new SSD and installed Ubuntu v20. Everything works, but I cannot boot into old Ubuntu v18 despite having such option in the GRUB menu.
I tried fixing it by boot-repair but it doesn't help.
Selecting option Ubuntu v18 returns:
error: no such device: [drive id here]
error: hd1 cannot get C/H/S values.
error: you need to load the kernel first.
My drive setup provided by boot-repair https://paste.ubuntu.com/p/B9rW6bxWXM/
Strangely enough it does work if I press F12 during the boot, manually confirm "Hard Drive" selection and THEN select Ubuntu v18.