i hade this problem a few times it's because os x somehow can't set the axtive flag which is needed by the bios/efi tho know at which disk thy should overgive the bootloader.
so you need to set the partition as active by hand via Terminal
- Go to finder/Spotlight, and launch Disk Utility
- Select the usb disk, and create a partition. Format it as MS-DOS FAT and select Options > MBR
- Go to the Terminal, and type the following
diskutil list
locate the name of your usb device. In my case it was rdisk2. Depending on your OS version it could differ.
diskutil unmountDisk /dev/rdisk2
Make the partition active and unmount the disk
sudo fdisk -e /dev/rdisk2
print
f 1
write
print
exit
diskutil unmountDisk /dev/rdisk2
have a look here for a colorful howto: http://tech.anoj.net/2013/01/fixing-unetbootin-on-mac-os-x-to-create.html?m=1