best practice is to dedicate an aggregate to the root. This is if you have an aggregate corruption later, you can get the system online much faster. Not sure if you can spare 3 disks, would be cheaper to use sata... I had noticed a bug with ACP causing excessive I/O on the root volume, disabled ACP at the time, not terribly interested in turning it back on. On root volumes that were on sata this was further exacerbated. Otherwise I haven't seen to much I/O hitting the root volume, I'd say go ahead with sata. I've had a 2040 I have in a branch office on the external sata for a while now.. no problems (also it's partner on the internal sas...) no problems of note...