He had writer's block once. It was the worst ten minutes of his life.

Jerry Pournelle, in "Chaos Manor: Is there an Upgrade in your future?" in Dr. Dobb's Journal : Software Tools For The Professional Programmer (2005), Vol. 30, Issues 374-379, p. 9

Most writers hate to write, and will grasp any excuse to do something else … There are exceptions. Isaac Asimov actually was never happier than sitting at a keyboard — first, his old typewriter; then, the TRS-80; and later, a more conventional PC. But then, Isaac was unusual, and his experience with writer's block was the worst 10 minutes of his life.