Its not enough to say 7200RPM. An older 7200RPM drive might average 50MBs easily. Newer ones would be closer to 70/80+, but if this drive is several builds old, then 50MBs might be what he's expecting. Tell us what drive you have, and the rest of your system specs.
First, 3Gbps is about 300MB/s.
Second, this speed is the speed of the SATA protocol so from "cache to host", not physical platter to host.
Third if you copy from and to the same disk, then you can halve this mark as it has to do both at once.