The default block size is 512 bytes which will work for most files and devices but the copy will be a lot faster if you use a larger block size. For example a floppy disk read with bs=1k count=1440 takes almost twice as long than if you use bs=1440k count=1. Don't make the block size too large because windows will run out of memory. 1M is probably a good size and upper limit. 1G works but is slow