When I’m trying to create bootable Linux stick on Mac OS X (yes, I’m doing it) I usually get this error on PC (during boot attempt):

Missing operating system

This is because Unetbootin fails to set partition on USB stick as active and also somehow (I’m lazy to do code review) fails to install mbr.bin from syslinux.

This is how I fix it:

1. plug USB stick to Mac
2. list devices:

diskutil list

3. unmount usb stick (no, eject from Finder is not right thing to do):

diskutil unmountDisk /dev/disk2

4. fix problem with active partition:

sudo fdisk -e /dev/disk2
f 1
write
exit
diskutil unmountDisk /dev/disk2

5. download syslinux

6. write mbr.bin to USB stick (locate mbr.bin first):

sudo dd conv=notrunc bs=440 count=1 if=mbr.bin of=/dev/disk2

7. write distribution as usual using Unetbootin.