Goto this page:
http://www.storagereview.com/comparison.html

Select SR File Server Drivemark 2002

You get a graph of all the disc drives ( including scsi and serial ata ( sata ) ) showing what the number of operations per second a drive is capable of performing.

Hard drive speed is complex and dependant on a number of factors - not just the headline quoted MB/s transfer rate.
The headline datarate is misleading.

Disc drives are not created equal.

Drives also have rotational latency and seek time issues which are much more important.
The rotational latency is because the drive is continuously rotating, and it takes a certain amount of time for the platter to move under the read heads.
This speed is related to the rpm of the drive.