Raspberry PI as NAS

A friend of mine was wondering about using a Raspberry PI as a NAS, the question being: how fast is the PI at file transfers from/to an external hard disk connected via a USB rack.

So I booted up my PI, mounted some random 3.5 inch Samsung 160GB SATA hard drive inside another random Spire USB-SATA external (powered) rack and gave it a go at some testing.