Copy ISO to Bootable USB Stick

22 February 2019

An .iso file is an image of a bootable disk. If we can can put it on a USB stick, we can boot our computer to that USB stick contents. Here's a way to do that in MacOS.

We find ourselves a fresh USB and clear it out. This can be done with the Disk Util.app, built into MacOS. Erase it and format it.

Unmount the USB Stick

Make sure it's unmounted, or later it'll show as busy when we try to copy files to it. We'll get an error like dd: /dev/disk3: Resource busy.

To unmount, check what mounted volumes we have:

ls /Volumes

Identify the USB stick's volume name, in my case named LINUXINSTAL, and type:

diskutil unmount /Volumes/LINUXINSTAL

Find USB Stick Drive Name

We found the name of the mounted USB stick, but we want to know the identifier of the USB stick as a drive, still plugged into our computer.

This can be done with:

distutil list

This will list various disks that are physically present on computer. Find the one with the size and filesystem type that seem to indicate your USB stick.

Mine looks like:

/dev/disk3 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *2.0 GB     disk3

Now that we can identify the destination, it's time to copy files over.

Copy the ISO to the USB Stick

To copy the .iso file, there's a handy built-in utility called dd, meant for moving and converting files.

To setup that command, type:

sudo dd bs=4m if=~/Downloads/myiso.iso of=/dev/disk3

That should do the trick. That command will likely take a while, differing based on the size of the file and the hardware involved.

To break down the command:

  • bs - sets the block size (this is how many bytes are written at a time)
  • if - the input file path (otherwise default is stdin)
  • of - the output file path (otherwise default is stdout)

Please note that for Mac, the bs unit has to be a lowercase m. Otherwise, you'll get Illegal numeric value. Also specific to Mac, you cannot add the argument status=progress. Otherwise, you'll get unknown operand status.

If you did it right, you're all set (or you will be given time!). Then give it the boot!