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Download Ubuntu Desktop
Open the Terminal (in /Applications/Utilities/ or query Terminal in
Spotlight)
Convert the .iso file to .img using the convert option of
hdiutil. Example:
hdiutil convert -format UDRW ~/path/to/target.iso -o ~/path/to/ubuntu.img
Note: OS X tends to put the .dmg ending on the output file
automatically.
Run diskutil list to get the current list of devices
Insert your flash media
Run diskutil list again and determine the device node assigned to
your flash media (e.g. /dev/disk2). Run
diskutil unmountDisk /dev/diskN
(replace N with the disk number from the last command; in the previous example, N would be 2)
Execute the following command while replacing
/path/to/downloaded.img with the path where the image file is
located; for example, ./ubuntu.img or ./ubuntu.dmg).
sudo dd if=/path/to/downloaded.img of=/dev/rdiskN bs=1m
Using /dev/rdisk instead of /dev/disk may be faster.
If you see the error dd: Invalid number '1m', you are using GNU
dd. Use the same command but replace bs=1m with bs=1M.
If you see the error dd:/devdiskN: Resource busy, make sure the disk
is not in use. Start the Disk Utility.app and unmount (don’t eject)
the drive.
Run diskutil eject /dev/diskN and remove your flash media when the
command completes.
Restart your Mac and press Alt while the Mac is restarting to choose the USB-Stick.