I'm trying to dual boot OS X and Elementary OS Freya on my Early 2011 MacBook Pro. I made a boot drive and followed these instructions (especially step 10) to boot but when I restart my computer and select my boot disk (I used a USB stick) I got the unhappy "No bootable device - insert boot drive and press any key" message. My machine then proceeds to not respond forcing me to use the hard shutdown. How can I make sure my machine recognizes the OS on my USB?
This sounds very much like the modifications you have done have wiped your boot block. Dual booting a Mac is never a good idea it is far better if possible to make use of the virtual drive software available this will give you a far better integration with your computer.
If a dual boot is necessary then check the Dual Boot Camp assistant to make sure of compatibility. Often people forget that older Mac machines will run dual boot, but the inside guts of the machine has been changed greatly to bring it all up to date for Mac OSX.
I would try and rescue your boot block if possible and your original layout on your drives partition should have been backed up prior to making these mods. I see it quite a lot in my business with Mac OSX and Windows as well. Making a backup of the partition on an external drive should be the first thing you do as a matter of course. Rescue may prove impossible depending on how deep the second OS has gone into the drive it may have wiped the original installation.
This advice and should be taken as such.