It looks like the problem is solved. I just disabled the "fast boot" (or sth equivalent) option at the EFI motherboard menu during boot process. As a result of the optimization EFI made to speed up booting, incomplete device map seems to be passed to bootloader - with information about just the target OS hard disk.

GRUB used to see all the disks even with the option being on if I entered the EFI menu and chose to boot GRUB from it, so the optimization only affects direct boots, those avoiding the menu. May be sensitive if you have large '> 1 TB' disks with numerous partitions for EFI to look through.

It looks like the problem is solved. I just disabled the "fast boot" (or sth equivalent) option at the EFI motherboard menu during boot process. As a result of the optimization EFI made to speed up booting, incomplete device map seems to be passed to bootloader - with information about just the target OS hard disk.

GRUB used to see all the disks even with the option being on if I entered the EFI menu and chose to boot GRUB from it, so apparently, the optimization only affects direct boots, those avoiding the menu. May be sensitive if you have large '> 1 TB' disks with numerous partitions for EFI to look through.