could you detail how to activate the prune mode please ?
I've specified it in my post actually. Go to Settings>Options at the top and you should see the options to change the config. You can access bitcoin.conf through there too and know which configs are active.
Basically I'd just need to transfer the bitcoin folder which is currently in Application support to an external harddrive ? Then I can delete the one in my desktop without any risk ?
Always backup your wallet.dat first. After moving your data directory, you have to modify the config file to specify the new path of the data directory to follow. Or else, the client will simply make a new data directory in the original place and start synchronizing again.