ubuntu iso to bootable usb

Open a Terminal (under Utilities)

Run diskutil list 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 sudo dd if=/path/to/downloaded.iso of=/dev/diskN bs=1m (replace /path/to/downloaded.iso with the path where the image file is located; for example, ./windows7.iso)

Run diskutil eject /dev/diskN and remove your flash media when the command completes (this can take a few hours on slower drives)

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1
Download Ubuntu Desktop

2
Open the Terminal (in /Applications/Utilities/ or query Terminal in Spotlight).

3
Convert the .iso file to .img using the convert option of hdiutil e.g.,

hdiutil convert -format UDRW -o ~/path/to/target.img ~/path/to/ubuntu.iso
Note: OS X tends to put the .dmg ending on the output file automatically.

4
Run
diskutil list
to get the current list of devices.

5
Insert your flash media.

6
Run
diskutil list
again and determine the device node assigned to your flash media (e.g. /dev/disk2).

7
Run
diskutil unmountDisk /dev/diskN
(replace N with the disk number from the last command; in the previous example, N would be 2).

8
Execute
sudo dd if=/path/to/downloaded.img of=/dev/rdiskN bs=1m
(replace /path/to/downloaded.img with the path where the image file is located; for example, ./ubuntu.img or ./ubuntu.dmg).
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: /dev/diskN: Resource busy, make sure the disk is not in use. Start the 'Disk Utility.app' and unmount (don't eject) the drive

9
Run
diskutil eject /dev/diskN
and remove your flash media when the command completes.

10
MAC
Restart your Mac and press alt/option key while the Mac is restarting to choose the USB stick.
WINDOWS
change boot device order in BIOS (hold down f2 during startup)