Macbook does not recognise bootable usb disk carrying 'Lubuntu 14.04.1 desktop amd64-mac.iso'

i want to install lubuntu-14.04.1-desktop-amd64+mac on a macbook 3.1 that currently has ubuntu 17.04.

i created a bootable usb (sandisk 16GB) using start up disk creator on a dell inspiron that runs ubuntu 16.04. i checked the usb on the dell and it booted just fine. i tried lubuntu without installing it.

i did the exact same thing on another usb (kingston 1GB) and again checked it out on the dell. worked fine too

when i plug either usb drive on the macbook and boot holding the 'option' key down i end up on a 'boot menu' screen with only a 'windows' named hard drive as an option...

any ideas why the macbook refuses to 'see' the usb disks???

thanks a lot