Yes, it is possible to raid together different size drives, but you will only have twice the smallest drive in the array. For example, say you bought a 10 gig and a 15 gig, raided them together, and you would end up with a 20 gig array. The same holds true for larger arrays too, unless you are panning(sp?), which if you are, you pretty much lost the advantages of raid systems and are wasting your time.