In simple terms a write blocker sits between the original drive or device and your forensic workstation and stops any write commands from getting through, while letting you read every bit. Along with checksums, write blockers help maintaining chain of custody, ensuring that the imaging step is repeatable and verifiable. Hardware write blockers are physical devices you plug the source media into, software write blockers are installed on the acquisition system, and the former type is more robust than the latter. The point, however, is not speed or convenience, it is to make a forensic copy without changing the original, so the evidence you work from is the same as the evidence you seized. And here comes the question: which laws or standards mandate using a write blocker?