If you bought an HDD in the last couple of years, some chances that your HDD's performance is crippled by the Automatic acoustic management (AAM) feature. AAM is a method for reducing acoustic emanations (noise reduction) and is popular especially among Western Digital HDDs. You can use software like EVEREST or SiSoftware Sandra to determine if your HDD uses AAM. Anyhow, AAM reduces performance in exchange for acoustics.