Hello I have actually successfully created a live Ubuntu USB (12.04.3 64-bit on 8GB LiveUSB) using unetbootin, however it is not persistent and will not boot on a Mac.
Or, if anyone has any suggestions on how to edit the .iso file so it boots from a Mac and is persistent, that would be great too. I am running OSX Mavericks so the application would have to run on Mavericks too.
I tried following http://ubuntuforums.org/showthread.php?t=2174630 ans, as suggested below, chose GUID partition (not MBR) and then add persistent word in grub.cfg file. and after I booted and changed a few settings, I tried to reboot using the restart option in the ubuntu menu, and it didn't work. I manually powered my computer off and on, but when I tried to boot the second time, I got past the Ubuntu logo and then my screen went black. I tried this some more times and got the same result. here is my text from the grub.cfg file:
if loadfont /boot/grub/font.pf2 ;
then set gfxmode=auto insmod efi_gop insmod efi_uga insmod gfxterm terminal_output gfxterm
fi
set menu_color_normal=white/black
set menu_color_highlight=black/light-gray
menuentry "Try Ubuntu without installing"
{ set gfxpayload=keep linux /casper/vmlinuz.efi persistent file=/cdrom/preseed/ubuntu.seed boot=casper quiet splash --
initrd /casper/initrd.lz }
menuentry "Install Ubuntu"
{ set gfxpayload=keep linux /casper/vmlinuz.efi file=/cdrom/preseed/ubuntu.seed boot=casper only-ubiquity quiet splash --
initrd /casper/initrd.lz }
menuentry "Check disc for defects"
{ set gfxpayload=keep linux /casper/vmlinuz.efi boot=casper integrity-check quiet splash --
initrd /casper/initrd.lz }