That single drive looks like it is using a single 320GB platter.
Against the Raid 0 drives in your lappy which use either 160GB or 80GB platters.
Thats why for a long time i was only running one drive as often a 320GB platter will out perform two 160 GB HDDs in Raid.
Everything looks normal, its just showing up Raid isnt all its cracked up to be.
Now two 320GB platter drives in Raid is a different matter, which is what i have just upgraded to.