I have an HP ML150 with SATA Raid configured. Windows 2003 server boots from that raid configuration. I recently added an external USB drive to the server and now it tries to boot from that device unless I unplug it when the server is posting.
I went through the bios settings and the new external hard disk shows up under hard disks when I expand that boot option along with my raid array.
My boot order looks like this.
Tandberg (this is the external drive)
Notice that hard disks are expanded and you cannot move something up or down in that expanded list. I haven't checked Tandbergs website yet.
Here is the product http://www.rdxquikstor.com/