Grub (Os-prober) can't find Windows 10 on another disk

[winnie]

This might be related to your issue. Setting the correct flags might resolve this:

After reinstalling windows bootloader, grub-update cannot find it

Maybe add the esp and boot flags to the EFI partition? sudo parted /dev/sda set 4 esp on sudo parted /dev/sda set 4 boot on And then try again with update-grub.

As an aside: Use parted instead of fdisk from now on.