dd — Copy, Clone and Wipe Drives Block by Block

dd copies data block by block straight at the device level — perfect for disk images, cloning whole drives, building bootable USB sticks or securely wiping disks. That same power makes it dangerous: dd has earned the nickname "disk destroyer" for good reason. Swap if= and of=, or point of=/dev/sdX at the wrong disk, and you overwrite an entire drive instantly and irreversibly — no prompt, no trash can, no undo. Always confirm the target device with lsblk before you press Enter.

Basic Usage

dd if=<input> of=<output> — Copy data from input file to output file.

dd if=/dev/sda of=/dev/sdb

dd if=<input> of=<output> bs=<size> — Copy with a specific block size (affects speed).

dd if=/dev/sda of=disk.img bs=4M

dd if=<input> of=<output> bs=<size> count=<n> — Copy only N blocks of the specified size.

dd if=/dev/zero of=testfile bs=1M count=100

dd if=<input> of=<output> status=progress — Show transfer progress during copy.

dd if=/dev/sda of=backup.img bs=4M status=progress

dd if=<input> of=<output> conv=<flags> — Apply conversion options during copy.

dd if=data.txt of=upper.txt conv=ucase

Disk Images & Cloning

dd if=<device> of=<image> bs=4M status=progress — Create a raw disk image from a device.

dd if=/dev/sda of=disk-backup.img bs=4M status=progress

dd if=<image> of=<device> bs=4M status=progress — Restore a disk image to a device.

dd if=disk-backup.img of=/dev/sda bs=4M status=progress

dd if=<device> bs=4M status=progress | gzip > <image>.gz — Create a compressed disk image.

dd if=/dev/sda bs=4M status=progress | gzip > disk-backup.img.gz

gunzip -c <image>.gz | dd of=<device> bs=4M status=progress — Restore a compressed disk image.

gunzip -c disk-backup.img.gz | dd of=/dev/sda bs=4M status=progress

dd if=<device1> of=<device2> bs=4M conv=noerror,sync status=progress — Clone a disk (skip errors, pad with zeros).

dd if=/dev/sda of=/dev/sdb bs=4M conv=noerror,sync status=progress

Bootable USB & ISO

dd if=<iso> of=<device> bs=4M status=progress oflag=sync — Write an ISO image to a USB drive.

dd if=ubuntu-24.04-desktop-amd64.iso of=/dev/sdb bs=4M status=progress oflag=sync

dd if=<device> of=<iso> bs=4M status=progress — Create an ISO from a CD/DVD drive.

dd if=/dev/cdrom of=disc.iso bs=4M status=progress

dd if=/dev/zero of=<device> bs=512 count=1 — Wipe the MBR (first 512 bytes) of a disk.

dd if=/dev/zero of=/dev/sdb bs=512 count=1

Generate Test Files

dd if=/dev/zero of=<file> bs=1M count=<n> — Create a file filled with zeros of a specific size.

dd if=/dev/zero of=100mb.bin bs=1M count=100

dd if=/dev/urandom of=<file> bs=1M count=<n> — Create a file filled with random data.

dd if=/dev/urandom of=random.bin bs=1M count=50

dd if=/dev/zero of=<file> bs=1 count=0 seek=<size> — Create a sparse file (allocates space without writing).

dd if=/dev/zero of=sparse.img bs=1 count=0 seek=10G

Wipe & Secure Erase

dd if=/dev/zero of=<device> bs=4M status=progress — Wipe a disk with zeros.

dd if=/dev/zero of=/dev/sdb bs=4M status=progress

dd if=/dev/urandom of=<device> bs=4M status=progress — Overwrite a disk with random data (more secure).

dd if=/dev/urandom of=/dev/sdb bs=4M status=progress

dd if=/dev/zero of=<file> bs=4M; rm <file> — Overwrite a file before deleting it.

dd if=/dev/zero of=secret.txt bs=4M; rm secret.txt

Benchmark & Performance

dd if=/dev/zero of=<file> bs=1G count=1 oflag=dsync — Benchmark write speed (sync after each block).

dd if=/dev/zero of=/tmp/testfile bs=1G count=1 oflag=dsync

dd if=<file> of=/dev/null bs=1M — Benchmark read speed (discard output).

dd if=/tmp/testfile of=/dev/null bs=1M

dd if=/dev/zero of=<file> bs=<size> count=<n> oflag=direct — Test direct I/O write speed (bypass cache).

dd if=/dev/zero of=/tmp/testfile bs=4M count=256 oflag=direct

Options & Flags

bs=<size> — Block size for read and write (K, M, G suffixes). Larger = faster.

bs=4M (common for disk operations)

count=<n> — Copy only N input blocks.

count=100 (with bs=1M = 100 MB)

skip=<n> — Skip N blocks at the start of input.

skip=1 bs=512 (skip MBR)

seek=<n> — Skip N blocks at the start of output.

seek=1 bs=512 (write after MBR)

conv=noerror,sync — Continue on read errors and pad with zeros (for damaged disks).

dd if=/dev/sda of=rescue.img conv=noerror,sync bs=4K

oflag=sync — Synchronize output after each write (ensures data is flushed).

dd if=image.iso of=/dev/sdb bs=4M oflag=sync

iflag=fullblock — Accumulate full input blocks (prevents short reads from pipes).

dd if=/dev/stdin of=output.bin bs=1M iflag=fullblock

Conclusion

dd is one of the most powerful — and most dangerous — tools in the Unix world: it writes blindly to exactly where you tell it, with no confirmation and no undo. A wrong of=/dev/sdX destroys an entire disk in seconds. So always verify the target device first with lsblk or fdisk -l, never swap if= and of=, and unmount the device before writing. Use status=progress for visible progress, a sensible bs= (usually 4M) for throughput, and conv=fsync or oflag=sync so all data actually lands on the medium instead of sitting in the cache. On macOS, address the raw device /dev/rdiskN (much faster) and unmount it first with diskutil unmountDisk. When in doubt, check one more time rather than overwriting a disk you can never get back.