I recently bought a REI Ultralite 45 pack. It is a pretty nice, lightweight pack. Although, I don't think that I would use it for a SAR pack but it would make a great three-day pack for fast hiking.

The CamelBak BFM, or a "Three-Day" pack would make a better SAR pack. Their size, material and design are better suited for the abuse and weight of SAR/Wildernes Medical use than typical hiking/camping backpacks. I dislike top-load packs for SAR work, I like to be able to open my pack and see everything quickly.
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"Trust in God --and press-check. You cannot ignore danger and call it faith." -Duke