When I hold the C key on startup on OSX Yosemite 10.10.5 (macbook pro retina 2015 with force touch), it doesn't boot to the USB. Holding alt/option doesn't show the USB drive, and changing boot order in system preferences doesn't work either, as the USB doesn't show up. Upon plugging the USB into my macbook while it's on and running OSX, I get the error message "The disk you inserted was not readable by this computer.", which should indicate that linux is indeed installed correctly onto the USB.
I'm able to boot to this USB using a windows (10) system.
Used the commands in this video to install to the USB: https://www.youtube.com/watch?v=sDq1oaYRgeg
Using Grub bootloader. Installed "archlinux-2015.11.01-dual.iso". Booted i686. Formatted both partitions of the USB to ext4.
My question here is, what's a possible reason for me not being able to boot to the drive, and how would I fix it?