You need to do this before the grey screen appears.
You can reset the pram by doing the following: If you've already tried the above solutions which I assume to have, try resetting the PRAM: Sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia -volume '/Volumes/MyVolume' -applicationpath /Applications/Install\ OS\ X\ Yosemite.appīe sure to replace /Volumes/MyVolume with your USB drive which you can find with ls /Volumes Running the command in Terminal where you have 'Install OS X Yosemite.app' in your applications folder should create a bootable USB drive. I'm not sure if the below command is what you refer to here. However on the MBP 2009 running 'EL Capitan'I am not given the option to boot from.
I tested the USB on my 2011 iMAC and can view it on that 'Startup Manager'.
I used the file command in Terminal from APPLE (for Yosemite) which all appears to be working. I have formatted the USB to Mac OS (Extended Journal) and made 1 Partition in the GUID format. The below command should handle all of that for you though. The USB drive should have an EFI partition, which contains files needed to make the drive bootable, in addition to the MacOS formatted partition containing the actual system to boot.
