So I was customizing a Lubuntu 18.04 install for a 2007 MacBook (model identifier MacBook2,1) to get the best performance out of it. It's the best compromise between performance and light in resources OS for these devices.
Because that device's fan is dead and it turns off due to overheating, I ordered a new fan and decided to do all the Lubuntu setup on another device, a slightly newer 2009 MacBook (model identifier MacBook4,1), and then put the HDD in the older one.
So I created a bootable USB installer which was immediately recognised by the 2009 MacBook and I was able to install it.
I set everything up I wanted, then put the HDD in the 2007 MacBook, and... it does not show up in the boot menu.
Of course, I thought my install was problematic or just that the HDD swap would not work, so I plugged my USB installer in it... would not show up either!
Both MacBooks work fine and the older one would even boot to a Windows HDD from another device.
I am stuck, and I don't want to have to dual boot from OS X.
Also, now that I know my installation is not the problem, I kind of want to get it to boot from that HDD because I did quite a lot of setup on it.
How can I resolve this?