How to copy a CentOS ISO to USB on Mac OS X

CentOS-LogoWindows users have a great free tool called ISO2USB which efficiently transfers ISO images to a USB stick. Mac users don’t have such a luxury – at least I haven’t found one yet.

Instead we can make use of a command line tool named dd which can do this for us. It needs a few parameters though, and in this article we’ll look at what those are. The following will work in both Mavericks and Yosemite, with ISOs from CentOS 6.5 and above. Our operation will result in a bootable USB stick.

First, head over to a CentOS Mirror and download your favourite ISO image. Next, have a USB stick handy and insert it into your Mac. Now open Terminal – it’s under Applications – Utilities, or do a Spotlight search to find it.

For this example I’m assuming that the image file is called centos.iso and that it’s in your Downloads directory. Let’s enter that directory by issuing the following command:

The second command lists all files, and your ISO image should be one of them.

Find out what device your USB stick is

So far so good, we’re in the right location. The dd command needs to know which file you want to copy (see above), and the device corresponding to your USB stick. It’s easy to get confused at this point. Let me explain this:

Devices are things attached to the system, such as hard disk, memory cards and USB sticks. Each device can hold a number of partitions which is where your data is stored. Depending on how many storage devices are attached to your system, you’ll get a specific address for each device. For example, /dev/disk0 is your internal Mac hard disk in which you’ll find three partitions.

Since our ISO image will overwrite all partitions on the USB stick, we need to know what the system knows our USB stick as. Type the following to get a list of what’s attached where:

Looks like I’ve got 5 storage devices on my system. The one at the bottom is my USB stick, an old 4GB model currently formatted with FAT32. Your layout will be different, so keep an eye on the SIZE parameter. If your currently formatted stick is named you can identify it that way too (mine is called C64).

The device I want to use here is /dev/disk4. Note that when we get to work, everything on your USB stick will be erased when we copy the ISO over. The dd command will not warn you before this happens.

Unmount your USB stick

Before we can continue we need to make sure your USB stick isn’t mounted to OS X. If it was formatted with a filesystem that your Mac can read, then you’ll see your stick as an orange icon on the desktop. But since the dd command will do a low level format with a different filesystem, OS X needs to let go of our stick. If we don’t do this, you’ll get a “Resource busy” message in the next step.

To unmount your stick, type the following:

That orange icon should disappear from the desktop, and you will no longer see your USB stick in the Finder either. Leave it attached though – do not eject it.

Copy the ISO image over

Now let’s give the dd command something to do. Since we’re in the correct directory already, type the following (you will be prompted for your password):

Amend /dev/disk4 with your own device. Feel free to specify the direct path to your image file in place of centos.iso (no wildcards I’m afraid).

This step can take a long time, during which you won’t get any feedback whatsoever. That’s normal, even though it looks like your session hangs. It’s all good. As long as you leave the Terminal window open, feel free to do other things with your Mac and leave it running.

In the above example I was copying a 190MB image and it took just over four minutes. I had an abysmally slow USB stick mind you – something to keep in mind when you want to transfer a large “everything ISO” image onto a stick from 10 years ago: you’re not doing yourself a favour, it’ll take hours to copy, and just as long to boot from later.

Should you want to terminate your dd session during this time (and use a faster USB stick for example), simply hit CTRL+C and you’ll be returned to the command line.

If all went well you’ll receive a summary message at the end. Now you’re ready to boot from your stick into the wonderful world of CentOS.

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13 thoughts on “How to copy a CentOS ISO to USB on Mac OS X

  1. Use a larger block size with dd to significantly increase your transfer rate.

    Even older, slow USB devices have a much higher supported transfer rate than the default dd block size 1byte. Reading and writing 1byte at a time has enormous OS overhead. change the block size to something around 1 megabyte and you’ll get the same task accomplished much more quickly.

    In your example:

    sudo dd bs=1m if=centos.iso of=/dev/disk4

  2. This is not working for me to create a bootable Centos 6.7 to install on a new Dell R730xd. I get a “boot failure : make sure you have a compatible media device installed” .. when I insert the USB Stick and choose it as the boot device. Going nuts here! ๐Ÿ™

    1. Hi Steve, if it would be easy, everybody would be running CentOS ๐Ÿ™‚

      I never had that problem myself. Here’s what I can suggest:

      • Did you try a different stick?
      • Did you try booting from the stick on different hardware (that’s safe, just make sure you don’t go through with the full installation of course…).
      • Last resort: if you have access to a Windows machine you can use ISO2USB:
      1. I have tried 3 different sticks — same issue. No other Dell type H/W available.

        However, I just made a break through — if I change the BIOS from “UEFI” to “BIOS” — it DOES boot .. but I need UEFI because I have 12TB of disk. So not sure what that means.. or what to do

  3. One quick tip, and thank you for the article, when the Terminal screen is open and running the dd process, press control-t to se progress. That will show you the current byte-level progress of the transfer of the ISO.

  4. I hit ctrl + t, it says the disk I inserted could not be read and asked if I wanted to eject and now ctrl + t isn’t working do i need to restart

    1. Where do you get that message, in the terminal window or from your Mac Desktop? The desktop may show you this message if the stick was previously formatted with a Linux partition. But since you’re unmounting the drive anyway, Eject will do that for you. To find out which device the drive was attached to, I’d use a stick your Mac can read, show the diskutil list and remember the slot. Then put the unreadable stick back in, let it eject (but don’t take it out), and proceed to copy the ISO image.

      If CTRL+T isn’t working, try CTRL+C. This should interrupt whatever Terminal is doing.

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