There is a couple of things.
Like Zygneri said there is a noticeable difference between 5400 and 7200 rpm drives. as far as drive access. Other things to consider, I know in the HR20 that it only had a Sata1 standard, while the chipset supported faster sata2 standard. Also if you have a higher end drive with prependicular recording vs standard write methods. Another thing is with things like Prependicular recording can take larger drives that use to span multiple platers down to a single plater. Less platers to search the faster the drive. A old 500gig sata 2 that is broken up on 3 physical platers ( the metal disk inside the drive it self ) vs a newer sata2 that now been able to reduce it down to 1 physical plater.