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Making a bootable USB from an .iso image on Mac OS X

updated 11 November 2015

Most bootable ISO images are designed to be burned to a CD-ROM. This guide walks through copying one onto a USB key instead, so any machine that boots from USB can use it the same way it would a disc.

Note: Intel-based Apple machines have an EFI BIOS that cannot boot USB keys. This process does not work for installing OS X onto Apple hardware.

Prepare the USB Drive

The first step wipes the partition structure on the USB key. This destroys all data on the key.

Open Disk Utility (located at /Applications/Utilities/).

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Then do the following:

  1. Select the USB key (select the root device, not its partitions)
  2. Select the Partition tab at the top
  3. Change the Scheme to 1 Partition
  4. Change the Format to Free Space
  5. Click Apply

A confirmation dialog will appear before any data is deleted. Click Partition to proceed. Quit Disk Utility once it finishes.

Wiping the drive ensures the USB key is in a known state and that OS X has not auto-mounted any of its volumes. You can skip this step if you unmount the volumes yourself first.

Preparing the ISO image

Convert the ISO to a Read/Write Universal Disk Image Format (UDRW) that can be written to the drive. The example here uses a Debian 7.1.0 net install ISO, but the process works for any ISO file.

hdiutil convert -format UDRW -o debian-7.1.0-amd64-netinst.img debian-7.1.0-amd64-netinst.iso

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The command creates the .img file at the output path. hdiutil appends a .dmg suffix automatically, so the output file will likely be named debian-7.1.0-amd64-netinst.img.dmg.

Copy the image to the USB key

Run diskutil list to identify the USB key’s device node:

diskutil list

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In this example the USB key is /dev/disk1.

Use the raw disk device (/dev/rdisk1 instead of /dev/disk1) to bypass the buffer layer and copy significantly faster. Also specify a block size of 1m for another speed boost.

dd if=debian-7.1.0-amd64-netinst.img.dmg of=/dev/rdisk1 bs=1m

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dd reads from the input file (if=) and writes to the output device (of=) block by block. Once the command completes, the USB key is ready to boot.