dd

Fantastic because it's able to copy bytes directly from a larger drive onto a smaller drive (until reaching the end of the smaller drive of course). This allows setting up a partition table that will fit on the smaller drive, on the larger drive, then copying it.

It just works. GUI utilities for writing ISO files (for example) are just not required and take longer to use than a simple commandline utility.

'dd' is a powerful command-line program that's been a staple of UNIX operating systems since the 1980s.

Pros:

• Ubiquitous on all UNIX-like operating systems
  • When you use Linux, macOS or BSD, 'dd' will be available.
• Extremely powerful options for advanced users
  • You can copy exactly blocks that want to exactly where you want (using the options like count=, bs=, seek=, skip=).
  • The manual is of course invaluable with this: man dd

Cons:

• Very esoteric interface
  • dd predates standard UNIX options syntax like mkdir -p, so the option syntax is weird
• No progress bar (see Tips section below)
• A single typo can destroy your data
  • Always use eg, fdisk -l /dev/sda to check your device node, because dd if=my_linux.iso of=/dev/sda could end up overwriting an important disk!
  • 'dd' is affectionately called "Disk Destroyer" for a reason!
• Default block size makes transfers very slow by default (see Tips section below)
• Not robust to bad sectors
  • The standard dd command is not very good at handling clusters of bad sectors within dying optical media and hard drives. For such disks, look at GNU ddrescue: GNU ddrescue

Tips:

• Set the default block size to vastly speed up your transfers
  • Try setting bs=1M to set a 1 megabyte block size and see if it speeds up your transfer!
• Consider periodically send a USR1 signal to request dd to print statistics
  • On Linux machines, after you start your transfer switch to a new terminal and simply run the following to make dd print statistics each second:
    • watch -n 1 kill -USR1 `pgrep dd`
  • On BSD and macOS, send the INFO signal

For users new to 'dd' this may all seem overwhelming. If all you're looking to do is flash a Linux ISO image onto a USB stick and move on with your life then there are definitely much easier applications.

But there is a reason dd has been around for so many decades. It's simply extremely powerful, versatile and ubiquitous.