Are you asking why your hard drive transfer rates are only in the 50-60MB/sec range?
If you are, the reason is simple - That's about as fast as they get.
SATA-2 offers 3Gbps (300mb/sec) at the interface, but the actual drive can only push about 60-70MB/sec tops currently. Burst rates may take advantage of the extra interface bandwidth, but no drive can sustain data transfer fast enough.
It's sort of like a highway with a 300MPH speed limit, but your car can only do 120MPH.