Mac user here. I have two laptops, a late 2010 MacBook Air (Intel Core 2 duo) and a late 2011 MacBook Pro 17" (Intel i7). Having read about elementaryOS Freya, I thought I'd like to try installing it on an external USB to try it out as an alternative OS.
I downloaded both the 64bit and 32bit versions, as well as UNetbootin, and have followed the installation instructions on the website, installing it on a 32GB thumb drive and on an USB-2 External. I tried the 64bit on the MBP with the thumb drive, and the 32bit on the MBA using the external HD.
In each case the process seems to run through normally, to the point where it says I should re-boot holding down the option key, but it then gives me the warning that the Mac won't boot from a USB (not actually the case) and when I reboot holding down the Option key, as the website explains, the USB drives are not found.
How to proceed?