If you have a raid group of 18GB drives, and add a 36GB drive, the 36GB drive becomes the new parity drive. The old 18GB parity drive becomes a data drive, giving you 18GB of space. After that, all 36GB drives you add give you the full 36GB of space.
(Caveat: This is how Netapp's RAID 4 works. With other vendors, YMMV.)
Bruce
Beware that if an 18G drive fails and the only available hot spare is a 36G drive, then only 18G of the 36G spare drive is used and 18G is wasted. This situation is permanent until the volume is destroyed, at least under DOT 5.3.x. Fortunately, hot spares are not tied to any particular volume or raid group, so one 18G hot spare can replace 18G drives from multiple volumes or raid groups.
Steve Losen scl@virginia.edu phone: 804-924-0640
University of Virginia ITC Unix Support