That's about right by my estimates. IF you were copying from one HDD unit to another (both SATA), the AVERAGE data transfer rate might be 60 to 90 MB/s. BUT you are doing that operation on ONE disk - copy off to RAM, then copy back to HDD. So achieving half that AVERAGE speed seems about right.
You need to realize that 3Gb/s or 300 MB/s, the speed "spec" for SATA II, is the MAXIMUM BURST rate that the HDD unit can send (or receive) from its cache RAM to the computer controller (and on to main RAM, or wherever). It is NEVER the long-term AVERAGE data transfer rate. That speed is determined mainly by the speed of the mechanical elements - disk rotation, head movement, etc. The 60 to 90 MB/s rate is typical of most SATA HDD's, and the BEST some tests have reported for mechanical drives get close to 150 MB/s, the MAX rate of the ORIGINAL SATA spec. The new Solid State Drives (SSD's) are a different story - much faster.