Mac OSX : How to burn an ISO image to a USB key

If you have tried to create a bootable USB key from an iso image in Mac OSX using Disk Utility then you have probably encountered the frustrating and almost Windows-like cryptic error message “Could not validate source – Invalid argument” error.

Looking in the system log via the Console app I suspect this is because internally DiskUtil tries to run /usr/sbin/asr to verify the image, which fails.

UDIF and NDIF are image formats used by Apple, and Disk Utility is pretty hopeless with anything that falls outside of these standards. The iso standard is short for ISO9660 and is a standard that defines the format of an image intended for burning to CD. Even after using hdutil to convert the image to UDRO (a UDIF Read-Only image) Disk Utility will still stubbornly refuse to help.

Disk Destroyer Duplicator to the rescue

Being Unix based, OSX has the command line dd utility available. Short for Disk Duplicator, dd is a block level reader/writer that makes raw copies from one file to another. But you want to copy the image to a device, right? That’s fine, because everything in the world of Unix/Linux is a file – even devices.

Informally referred to as Disk Destroyer, should you tell dd to output to the wrong device then your day is definitely going to be spoiled, so to avoid any mishaps we will make sure we know which devices on your system is your USB stick. You can determine this from the command line:

Each physical device will be listed as /dev/disk0, /dev/disk1, /dev/disk2 etc. Each partition on e.g. disk0, is listed as /dev/disk0s1, /dev/disk0s2, /dev/disk0s3 and so on. In the above example, /dev/disk0 is the main boot disk and /dev/disk1 is the USB stick. But there is a nicer way to be sure by typing:

It’s pretty clear from this output that /dev/disk1 is indeed my 8Gb Sandisk Cruzer USB drive.

The easiest way to prepare a USB for burning is to erase it. This can be done using Disk Utility (I knew it was useful for something) by selecting your USB drive (not any of its partitions) in the left hand pane, clicking the Partition tab, and creating a 1 Partition layout, with the Format Free Space. See screenshot opposite.

Once you have removed any partition data from the device, the diskutil listing will look something like this:

The dd command is a simple animal, and only needs a few parameters:

• if is the path to the Input File
• of is the path to the Output File
• bs is the Block Size (how big a chunk of data is read and written at a time). There is no real advantage to choosing a blocksize bigger than 1 megabyte.

Lets suppose we want to burn an image called sophos_usb.iso to the USB drive. The command will be

Note: We changed the device /dev/disk1 to /dev/rdisk1 – this is because in OSX each storage device has two references to it. disk[n] which is a block level, buffered device, and rdisk[n] which is the raw device – copying to the rdisk reference is about 20 times faster.

Entering the sudo command will require your admin password (due to the sudo – Super User do command). It will then take some time to copy your data and you won’t get any feedback from the terminal. As you can see, my 1Gb iso took about 90 seconds to copy.

That’s it – you should now be able to eject your USB drive and boot up the PC.

You can’t boot Macs from the majority of iso images as they don’t have the necessary EFI boot loaders – if you are trying to burn a linux iso, then take a look at Mac Linux USB Loader which we might look at another day 🙂