sudo dd if=/dev/sda of=/dev/sdb bs=4096
Depending on the size of the old disk, this can take 15 minutes to a few hours (perhaps even overnight if the old disk is large, I suppose). Sadly, dd does not give you a progress indicator; you just have to wait for the prompt to come back. You may notice the hard drive activity light blinking furiously. This indicates that the disk is being heavily read or written to depending on which disk you're looking at, which is expected. When the lights stop blinking, dd is probably done.