Complex sentence: This means that the time to read (or write) any particular piece of data is smaller than it would be on a disk, the controller simply waits for the data to appear under the proper head as the drum turns.
Split into simpler sentences: This means that the time to read (or write) any particular piece of data is smaller than it would be on a disk. The controller simply waits for the data to appear under the proper head as the drum turns.