MacBook disk choosing not available

Hi everyone. New guy here on the forum.

I just installed Lubuntu (XFCE) on an MacBook Pro 2010 13" laptop, which all went fine and dandy. I now want to try out a different Linux Ubuntu flavor but I can't seem to access choosing disks when holding down option at startup. No disks at all show up. The disks do however show up clearly within the Lubuntu installation itself, including the USB boot drive I have created.

I have tried changing the mounting options by to just include the USB drive but to no success.

Any ideas what might be the cause of this, and how to solve it?

Many thanks in advance.