2009 iMAC will not boot from USB
I have installed Linux Mint (from a USB stick) but I'd like to go back + set this system up as a dual boot between MacOS (ElCapitan) and Mint Linux, so I need to do some disk re-partitioning. Probably ambitious, but at present I can't get past the first step !
Using a Windows wireless keyboard, I'm holding down the Alt key on startup/restart (when should I let the key go?).
iMac, OS X 10.11