In considering the performance of hard drives there are typically two primary factors that you should consider. Those are access time and transfer rate. Access time is measured in the number of milliseconds it takes the drive to get to the first piece of information after it has been requested. These are generally in the range of high single digits to the low teens. A 6 to 8 ms access time is considered pretty quick, 9.5 to 10.5 is pretty typical and anything higher than 12 is beginning to be considered a little pokey.
Transfer rate is another significant factor in hard drive performance.
This is the amount of data that hard drive can push out. This is measured in megabytes per second. It is important to consider both what your needs are and what your machine is capable of when looking at this number. This is simply the number of times that the hard disk in your hard drive spins around in a minute. Generally the faster the spin rate the faster the drive will be able to transfer information.