Ubuntu on (only) MacBook External drive (part2)

[Grizzly]

Hi Again

After my user killed two Xenial "live" USBs (one stick & one SSD disk) and 
fried his MacBook's own Boot (fixed later), user now wants (thinks) that 
putting Xenial on an SDRam card that will live full time in the MacBook (flush 
mounted) is the answer

User says "Duel Boot" but that may mean different things to you me and a 
MacBook user

To me it means Grub menu and deciding which (installed) "OS" to boot

To a MacBook user it may mean press/hold option key at boot and choose which 
"Disk" to boot

To others I guess depends on what hardware "most" of their machines are

Now so I am prepared for whatever is required I think there are a couple of 
probable ways this could go (not sure how to achieve all of them though)

1) Live Xenial on SDRam with nothing on MacBook (choose at option key)

2) Xenial installed on SDRam with nothing on MacBook (choose at option key)

3) Xenial installed on SDRam with Grub on MacBook (Choose from Grub menu)

#1 is easy to do & test, but has update problems (AFAIK) that´s how user killed 
first two tries, though I´ve gone that route on USB sticks and updated fine (on 
PC) and its not a "Real" file system

#2 How to get this and be sure "NOTHING" changes on MacBook?

#3 Be sure that "ONLY" grub is on MacBook 

Any pointers / pitfalls please from someone who has done this and if any or all 
would work?