Coreutils dd

dd is a tool for converting/copying files (primarily system devices exposed as files)

Usage

# create fake disk drive
sudo dd if=/dev/zero of=mydisk.img bs=1M count=1200  # 1M at a time, 1200 times (1G)
sudo mkfs.ext4 mydisk.img

# NOTE: on FreeBSD, you'll need to create a virtual block device
#       for the image using:
#       mdconfig -a -t vnode -f /path/to/image.img -u 0  # link image to virtual device
#       mount -t ext2fs /dev/md0 /mnt                    # mount the virtual-device, instead of disk file
#       mdconfig -d -u 0                                 # unlink image from virtual device