"Just picked up..."
How long?
It takes a bit for the cache to do its thing. And that is only for Read speed, for that data that lives in the small cache space. Write still happens at regular 5400 HDD speed.
InternalIs this drive mounted internally or externally?
[Internal]
Which motherboard do you have?
Which Sata port is it connected to?
What mode is your Sata ports set on in the bios?
[External]
What device are you using to interface to the drive and which port on the PC?