How fast is a harddisk? Some people will answer the speed of the bus to which the harddrive is attached. Ultra ATA should give you some 133MB/s, Wide Ultra 4 or Ultra 320 SCSI should give you
320MB/s, but never in your life will you get that kind of speed from only one harddisk. So, what is the speed then.
To find this, you need to know two things about the harddisk : Latency and Seek.
Latency is the speed at which any next spot on a track of the harddrive is brought to the head. This means, while the head remains passive, Latency is influenced by the rotational speed and determines the speed
with which the harddrive can read consecutive blocks of data. Usualy, SCSI and ATA harddrives score very well at this. Some ATA harddrives will even outperform SCSI harddrives at this.
Seek is the speed at which any spot on the harddrive can be located. Including data that is stored on a different track. This includes moving the head of the harddisk.
This is typically something at which SCSI harddrives are much better then ATA harddrives.