Dual boot Linux and windows (separate disks) using Grub2 SHOULDN'T BE THIS TRICKY

ISSUE:

has the grub menu disappeared?

does your system boot straight into windows?

Explanation

Each disk has its own OS, and efI bootloader

1) Linux bootlader (xubuntu) is sda1

2) Windows 10 bootloader is sdb2

Now, the grub2 files that matter are:

/etc/default/grub

in which you need to make sure 

GRUB_TIMEOUT_STYLE=menu

and doesn't say 

GRUB_TIMEOUT_STYLE=hidden

This makes sure you see the grub menu

and

/etc/grub.d/40_custom

(see https://linuxvox.com/blog/grub-config-file-ubuntu/)

where we add our extra menu option for Windows 10

(see https://askubuntu.com/questions/661947/add-windows-10-to-grub-os-list)

viz:

type 

lsblk 

to identify your windows EFI partition, thenblkid /dev/sda1 

(with the partition /dev name) to find the UUID of the EFI (call that YOUR_WIN_EFI_UUID)

then FINALLY, the code to add in the 

/etc/grub.d/40_custom\

is

menuentry 'Windows 10' {
    search --fs-uuid --no-floppy --set=root YOUR_WIN_EFI_UUID
    chainloader (${root})/EFI/Microsoft/Boot/bootmgfw.efi
}

Now... if Windows insists on ignoring the grub menu and booting straight into windows after an update or repair

you may need to type this from windows:

open cmd (as administrator):

bcdedit /set {bootmgr} path \EFI\ubuntu\grubx64.efi

then restart