How to make a bootable USB thumb drive in Mac OSX from an ISO file
An ISO file or image is an archive file containing the exact contents of an optical disc such as a CD or DVD. It includes the entire disc’s contents including the file system.
I’ve previously discussed converting DMG files to ISO files on your Mac. A DMG is similar to an ISO in that it’s a mountable image.
Usually ISOs are used to distribute CD images such as distributions of operating systems such as Linux. If you would really like to learn more about ISO files you can view this comprehensive Wikipedia page.
ISOs are not however limited to such uses and software is available to create your own ISO archives with just about anything on them.
One of the main advantages of an ISO is that because you are creating an entire image of a CD it takes everything with it including that CD’s ability to boot for example.
So if you normally boot from a CD such as your Windows XP CD or a Linux live CD then you can theoretically create an ISO image of these CDs and with the instructions below you can create a bootable USB thumb drive also which can be very handy in today’s netbook world.
Creating an ISO from a CD
You probably already have an ISO that you’d like to make a bootable thumb drive from but if you don’t, it’s quite easy to make one from a bootable CD on your Mac.
- Insert the CD
- Open Disk Utility
- In the left pane, highlight your CD/DVD drive
- On the toolbar, click New Image
- In the dialog box
- In the Save As field enter a filename for your image
- In the Image Format field, select CD/DVD master
- In the Compression field, select none
- Click Save
- After about 10 – 20 minutes, the creation process will be complete.
- You will notice however that the file name ends with .cdr but you can change this to .iso by simply renaming the file.
Creating the bootable USB thumb drive
It is technically possible to create the bootable thumb drive using Disk Utility but for this I will show you how to do it the more reliable way (in my opinion anyway, but hey that’s why you’re here) using Terminal.
- Plug in your USB drive and wait for it to mount. Make sure the capacity of the USB drive at least 1GB.
- We will be erasing the contents of the USB drive during this process so now is a good time to go ahead and copy anything you need off the USB drive
- Open Terminal
- Type the following and press enter: diskutil list
- You will see a table of data. This is a table of all the disks and volumes on your Mac. Look for the disk path for your usb drive. It will look something like /dev/diskx where x is the disk number. I’m pretty sure it won’t be zero as that’s usually your hard disk. In my case it’s /dev/disk2
- Type the following and press enter: diskutil unmountDisk /dev/diskX (change X to the disk number of your USB drive)
- OK for this next step I’ll teach you a quick and dirty trick. You must replace the letters YOURISOFILE with the path of the actual ISO you created in the previous section. Instead of finding the path and typing it into Terminal simply do the following:
- Type: dd if=
- Now drag your ISO file from your Mac’s Finder or Desktop or where ever you saved it and drop it on the Terminal window – no, I’m not crazy. You’ll notice when you drop the file that the path of your ISO will be inserted nicely after the dd if= that you just typed.
- Now finish typing the command: of=/dev/diskX bs=1m
- So the whole command you need to enter is: dd if=YOURISOFILE of=/dev/diskX bs=1m
- Remember to replace YOURISOFILE and diskX with your own values!!
- All set? Press Enter and wait – mine took 71 minutes to complete. When the process is finished you are returned to the Terminal prompt and hopefully you will have a bootable USB thumb drive.