We have a client who has an external RAID5 array using NSS. They are
currently 15krpm drives and we are looking to add a second array on the
other channel but are debating on 15k versus 10k drives(due to the 300GB
drives only being 10k). My question is, how does NSS handle writing data
to the two different devices within the pool?

I would assume that it would keep a file entirely on one device and not
write portions of a file to each partition as opposed to splitting data
between the two devices. Can anyone shed some light on this?