From Apple Stack Exchange
Open DiskUtility. Erase flash drive. Reformat as MS-DOS(FAT) with GUID partition.
Run hdiutil convert -format UDRW -o /path/to/target.img /path/to/ubuntu.iso
(May create a file .img.dmg which is ok.)
diskutil list
diskutil unmountDisk /dev/diskN
sudo dd if=/path/to/downloaded.img of=/dev/rdiskN bs=1m
diskutil eject /dev/diskN