I’ve found dd extremely useful when dealing with older or otherwise inconvenient disk formats.

This was particularly handy for Mac OS Standard / HFS media because OS X 10.6 Snow Leopard treated HFS as read-only.

I was also using the same technique for things such as Raspberry Pi boot media containing Linux filesystems that OS X did not natively understand.

Because dd works at the block-device level, it does not need the Mac to understand the filesystem before making an image.

There is one extremely important catch:

if you give dd the wrong output device, it will happily overwrite the wrong disk.

Always identify the device again with diskutil list, unmount it and verify the disk number before pressing Return.

Do not copy the example disk number literally.

Copy a device to an image file

diskutil list
diskutil unmountDisk /dev/diskN
sudo dd if=/dev/rdiskN of=~/test.img bs=1m
diskutil eject /dev/diskN

Restore an image to a device

diskutil list
diskutil unmountDisk /dev/diskN
sudo dd if=~/test.img of=/dev/rdiskN bs=1m
diskutil eject /dev/diskN

Replace N with the actual disk number you verified using diskutil list.

On the Mac, using the raw-device form such as /dev/rdisk3 can also be considerably faster than accessing /dev/disk3.

I tried Paragon ExtFS around this period as well and had bad experiences with corrupted media, so I stopped using it. That was my experience with my setup, not a general finding about every version of the software.

By 2015 I had also started using ApplePi-Baker for Raspberry Pi images because it wrapped this sort of workflow in a considerably friendlier interface.

The command-line method remains useful.

Just check the disk number.

Then check it again.

Sources