Creating a bootable Linux distro compatible with Mac OS X I have downloaded the appropriate Linux distro (in this case Fedora Xfce Live) and I have partitioned the USB with Disk Utility and attempted copying both the Fedora iso, and a Fedora iso converted to the Mac .img format.

When I booted (holding option key) the USB with Fedora didn't show up in either case in boot manager.

What needs to be done for Mac to consider the USB "bootable" and for it to be visible in "boot manager" when holding option key?
A working approach to get Fedora Xfce Live booting is the dd-method:


Download Fedora 23 Xfce Desktop.
Attach the thumb drive. The thumb drive will be erased!
Open Terminal and enter diskutil list to get the disk identifier of the thumb drive (e.g. /dev/disk3).
Unmount the thumb drive with diskutil umountDisk /dev/disk3
Enter:

sudo dd if=/Users/user_name/Downloads/Fedora-Live-Xfce-x86_64-23-10.iso of=/dev/rdisk3 bs=8m


This will copy the content of the iso to the thumb drive. When the process is finished you get an response like this one:

114+1 records in
114+1 records out
960495616 bytes transferred in 5.920656 secs (162227899 bytes/sec)

Exit and close Terminal.
Open System Preferences -> Startup Volume and choose "Fedora Live" (or similar).
Reboot your Mac. You may need a USB keyboard/mouse because Bluetooth devices like the Apple Magic Mouse initially don't work booted to Fedora Live.