Try this
Stick your current ubuntu usb into the usb port on the mac
hold down the option key and power on
you should get 2 drives showing.
use tab button to pick the usb - should be labelled as efi with a usb logo
press return key
it will then start to load.
It might stop - just wait a couple of mins and see if it loads
Click on the Ubuntu home icon
click on other locations
can you access the drive. - If not what is the error - take a pic
Stick your current ubuntu usb into the usb port on the mac
hold down the option key and power on
you should get 2 drives showing.
use tab button to pick the usb - should be labelled as efi with a usb logo
press return key
it will then start to load.
It might stop - just wait a couple of mins and see if it loads
Click on the Ubuntu home icon
click on other locations
can you access the drive. - If not what is the error - take a pic