Not sure I understand the question. The mechanical drive would have to be just as fast 520mb/sec. Unless you mean spin speed, and thats somewhat unrelated.
What you might be questioning is megabits vs megabytes. Megabit should be written as Mb, and Megabyte is MB. But in reality there is a ton of misuse of the correct capitalization and ways to write it. I'm sure I've screwed it up a few times on these boards myself. So in a bit to byte conversion, just divide the bit number by 8.