Question about Dual Boot at LattaPanda (mac/ubuntu)

i have 2 nvme drives here,
booth systems are installed on the panda: 1st Ubuntu, 2nd mojave.

  • i have a external drive from nvme to usb3.0

now can i say in clover to boot from that external drive ?
i see the drive in clover boot screen as U but it doesn’t boot… comes only a black screen.

If i put the ubuntu drive directly to the m2.key m slot booting is fine.