I know that doing a dd if=/dev/hda of=/dev/hdb does a deep hard drive copy. I've heard that people have been able to speed up the process by increasing the number of bytes that are read and written at a time (512) with the "bs" option. People have suggested that the optimal byte size is due to sector size. I personally think it would have something to do with the amount of cache that the hard drive has.

My question is:

What determines the ideal byte size for copying from a hard drive?

and

Why does that determine the ideal byte size?