My recent experience installing ubuntu 18.04 on same MacBook Pro mid 2009 ( MBP 5.5 ) that had ubuntu 14.04 and for THOSE WHO HAPPENS TO HAVE A BEAUTIFULL USELESS, WOTHLESS MacBook Pro that sits around doing nothing because Mountain Lion is overpass not suppoorted and dead!
My recipe is if you DO NOT GIVE UP AND ARE VERY PERSISTENT.
I have to say It is becoming tedious or I am getting older....
First let us be clear, The ''refind'' approach did not work before and not this time again for me on this Mac Book Pro and not on a more recent Mac Boo Pro with intel i5! and not with my older big MacBook Pro 2008 with 2 Zeon cpu totalling 4 cores.
On the MAc BOOk Pro mid-2009 I tried the DVD approach and pressing C key - No success it is always rejeted.
I tried pressing the ''alt-option'' key - not much luck it would stop after starting to read the 18.04.iso DVD, then engaged in a reject... and same with a usb flash drive alone.
The approach I used that was succesfull...
I got get rid of refind , insert the DVD + the usb flash drive all at same time.
Restart pressing ''Alt-option'' key + power on switch.
I Held on, cause it takes time. Don't let off the ''alt-option'' until you see the new choice showing ''windows'' and the ''yellow usb drive''.
Select the new yellow usb drive. It should go on... with the installaton process.
The first pitfall I had to face was at the time of selecting the city/country.
My choice was Montreal,Qc ... but that never stayed selected and it always moved to Toronto, On ... Why? unknown.... After reselecting over 20 times I decided to keep Toronto and go on...
Then came another problem with language. In all previous installations since Ubuntu 6.XX (over 10 years or so) I always seleced English US as the language
and things would roll on to install. This time, NO it cut off and died out. I had to restart the whole process from the beginning. I found out it was a NO NO.
By luck or chance after 5-6 times, I decide to selected ''English Canadian'',''Canadian Multilingual'' and regional ''Canadian French''. That is the only way it proceeded to install. I selected the minimal, with no downloads while installing. When I rebooted I could connect to update and dist-upgrade + Install all the other apps as I liked to choose.
There must be some bugs in this new vesion 18.04-1 and I pass my experience because many may have tried to install a much better OS on these Apple products the company wants us to throw away .Apple want us to Keep up piling the waste on the planet...and give them money