tried defragging and chkdsk? Doing these regularly will speed up your disk. Also- Sandra is NOT very reliable at benching harddrives. Your score will change depending on how much data you have on that drive, what filesystem and what size the datum are. Which makes no sense- your drive doesn't get weighed down by more data or anything

Sandy seems designed more as a bench of your filesystem as a whole. Like- if you compared two exactly identical drives, but one had a slightly larger capacity, the larger drive would have a higher score. This is sorta true, as densities increase so does read speed, but not to the extent Sandra says.
If you want something more accurate, more of a test just of the drive itself, try a search for ATTO Disk Bench.