Create Bootable USB Disk with DVD ISO Image (Mac)

You can create a bootable USB disk from a DVD ISO image on a Mac by using the terminal to unmount the drive and then writing the image directly to the raw device node. This process bypasses the need for third-party GUI tools or specific application folders, relying instead on standard Unix utilities available in macOS.

Preparing Your Tools and Drive

Before you begin, ensure you have a USB flash drive with sufficient capacity to hold the ISO file. It is critical to back up any data on this drive, as the creation process will erase all existing content. You also need the .iso file you wish to write. While many users look for graphical applications, the command line approach is often faster and more reliable for system-level tasks. First, insert your USB drive into a USB port on your Mac. Open the Terminal application, which you can find in /Applications/Utilities/. The first step is to identify the correct device identifier for your USB drive. Run the following command:

diskutil list

This command outputs a list of all storage devices connected to your Mac. Look for your USB drive in the list. It will typically appear as /dev/disk2, /dev/disk3, or similar, depending on how many other drives are connected. Note the identifier carefully. Do not select the partition (e.g., /dev/disk2s1), but rather the whole disk (e.g., /dev/disk2). Choosing the wrong disk can result in data loss on your internal hard drive.

Unmounting the USB Drive

Once you have identified the correct disk identifier, you must unmount it before writing to it. Unmounting makes the filesystem unavailable to the operating system while allowing low-level access to the block device. Replace diskN with your actual identifier.

diskutil unmountDisk /dev/diskN

For example, if your drive is /dev/disk2, the command would be:

diskutil unmountDisk /dev/disk2

If successful, the terminal will return "Unmount of all volumes on diskN was successful." If the drive does not unmount, it may be in use by another process. You can try closing any Finder windows showing the drive or restarting the computer before trying again.

Writing the ISO Image

The core of the process involves using the dd command to copy the ISO file byte-by-byte to the USB drive. This method ensures that the boot sector and partition table are preserved exactly as they are in the ISO. Use the following command structure:

sudo dd if=/path/to/your/image.iso of=/dev/rdiskN bs=1m

Here is a breakdown of the flags:

• if: Input file. Specify the full path to your .iso file.
• of: Output file. Specify the raw disk device (/dev/rdiskN instead of /dev/diskN is significantly faster because it uses raw I/O).
• bs=1m: Block size. Setting this to 1 megabyte speeds up the transfer considerably compared to the default block size.

For instance, if your ISO is located on the Desktop and named ubuntu.iso, and your USB drive is /dev/disk2, the command would be:

sudo dd if=~/Desktop/ubuntu.iso of=/dev/rdisk2 bs=1m

You will be prompted for your administrator password. Note that no characters will appear on screen as you type the password for security reasons. Press Enter after typing it. The command will not display progress bars by default. This can make it seem like the process has frozen. Be patient. Depending on the size of the ISO and the speed of your USB drive, this may take several minutes. Once complete, the terminal will return to the prompt, displaying statistics about bytes transferred and time taken.

Ejecting and Verifying

After dd finishes, it is good practice to sync the buffers to ensure all data is written to the disk. Run:

sync

Then, eject the drive safely using Disk Utility or the terminal command:

diskutil eject /dev/diskN

Your bootable USB is now ready. You can verify its functionality by restarting your Mac and holding the Option key to select the USB drive from the boot menu, or by connecting it to another compatible machine. For those interested in optimizing other aspects of their Mac setup, such as network configurations, you might find Setting SOCKS Proxy in torrc for MacOS useful. Additionally, if you need to manage storage efficiently alongside these hardware tasks, consider reading Optimizing Microsoft Remote Desktop (Windows App) Storage on Mac.

Frequently Asked Questions

What if my USB drive is not detected?

Ensure the drive is properly seated in the port and try a different USB port if possible. Check diskutil list again; if it still does not appear, the drive may be faulty or incompatible with macOS.

Why use /dev/rdisk instead of /dev/disk?

Using the r prefix accesses the raw disk device, which bypasses the buffer cache and results in significantly faster write speeds compared to the character device interface.

Can I use this method for Windows ISOs?

Yes, the dd command works for any valid ISO image, including Windows installation media. However, ensure your target machine supports the boot mode (UEFI vs Legacy BIOS) required by the ISO.

How do I know when the process is finished?

The terminal will return to the command prompt once the operation is complete. You can also monitor the process by opening a second Terminal window and running sudo kill -INFO [PID] where PID is the process ID of the dd command.

Is it safe to run this command without sudo?

No, writing to a disk device requires root privileges. Attempting to run the command without sudo will result in a permission denied error.