Is the 5.9 for your drive or your overall system rating? If it's for your drive, did you get the latest drivers for it? It might still be running under older, compatible drivers that aren't allowing it to perform at it's top speed.
If the drive is rated higher but your overall system experience is 5.9, remember. The Windows Experience overall rating takes on the LOWEST experience rating reported. For example, if your drive is rated at 6.5 but you have memory that has the lowest rating at 4.1 over all your other hardware, then you're overall rating is 4.1, even though you might have a lot of other hardware that's rated higher than that.