Large backpacks

I've been looking for the same thing and the ULA Catalyst seems to fit the bill. Has anyone here used one? ULA claims it weighs 43 oz., holds 4,600 cu. in./75 liters and is good for up to 40 lb. List price is $225.
http://www.ula-equipment.com/catalyst.htm

The Osprey Aether 70 is a nice pack and a little over 4,000 cubic inches. Its weight is probably closer to 4.5 lbs. or thereabouts. They also have some larger packs in their line that are not outlandishly heavy and the Osprey Atmos 65 is in that ballpark capacity wise but weighs a little less than the Aether.
Gerry Magnes

I'm going to use a Golite Pinniacle pack this summer to carry 35lbs of gear for my wife and daughter for a 3day 2 night trip in RMNP. It is rated to 40lbs and is 4500 cu. I have used it for winter camping and carried 24lbs very comfortably. it weighs about 25oz.

If it turns out that you are interested in the Aether, I have one and can answer any questions you might have. If your still interested, I would happily part with mine. It's a great pack, but my kids are grown and the pack is too big for my current gear. It carries very well. I've had upwards of 65 lbs in mine. Said with an odd mixture of pride and shame :).

I have a MountainSmith Specter which was the ultralight backpack of choice in 2001 by BackPacker's magazine, it weighs about 4 lbs and has the weight and capacity features you need. Only been used twice — no more than 2 weeks's amount of use. In super, superb shape. Will sell for $70 plus buyer pays shipping. (New was $270).

PM me if interested.

http://www.ultralightbackpacker.com/items.html#Backpacks

Mountainsmith Specter (5600ci/91.8l, 4lbs 5ozs) – For winter camping and mountaineering, this is my new Yeti pack. Born of the weight-friendly older Mountainlight series, the Specter and its smaller siblings are about the lightest, strongest packs you'll find if you require a full suspension system for heavy loads. If you have faith in Backpacker Magazine, their 2001 Editors' Choice awards were given across the entire Mountainlight-derived series.

http://www.ultralightbackpacker.com/items.html#Backpacks

Mountainsmith Mountainlight Packs
These are the most comfortable lightweight load-carriers we've tested.

http://www.backpackinglight.com/cgi-bin/backpackinglight/mountainsmith_specter_gg.html

A nice forum post on this pack:

http://www.backpackinglight.com/cgi-bin/backpackinglight/xdpy/forum_thread/429/index.html?skip_to_post=2293#2293

"J R ( RavenUL ) not the lightest… but my fav! on 04/01/2005 12:13:13 MST "

"I'm a big fan of MountainSmith. Their MountainLight line isnt the lightest out there, but they have a VERY functional waistbelt, good frames, and still have a good weight to CI ratio, and a good durability to cost ratio.

When carrying anything more than a base weight of about 10lbs (or longer than 3 days), I go with my MS Ghost. Its an excellent pack, but maxes its comfortable weight at about 35lbs, however for heavier loads their are a few beefier options – I like the Auspex, and it sounds like it would fit your needs well (light base load, but need to haul alot of water). If you foresee a need for an even beefier pack, the Specter is a very good pack as well – with a quick detach day pack for side trips if you so wish… both of these packs will ride well with 45+ lbs loads, but weigh a good 2 – 4 lbs less than other framed packs of similar dimentions and quality."

A pack from mchalepacks.com would not cost too much amortized over a whole family. It would be better built than a mtn smith.

The first is off the shelf: a ULA Catalyst. One of these with any size frame will handle what you specify, and more. With an XL frame and the dual framesheet option, just over 3 pounds. A great pack.
The second is for the pack of your dreams: Design your own with Ron Rod of R2 packs, http://www.r2packs.com. Check out Will Reitveld's article in BPL. I worked with Ron to build a ski pack, and it's the best pack I've ever owned. The fit alone is worth the bespoke price.

Just thought I'd give my input about the Osprey Aether line. I have used an older generation, before they gained weight, aether 60 for years. It is very tough, virtually waterproof, and carries heavier loads very well. The main feature I would highlight about these packs is the ability to make fine adjustments to the torso length. The shoulder harness attaches between two layers of velcro (it holds firmly) and you can slide it up and down a little at a time to really dial in the fit. Having such a good fit allows you to make small adjustment to the shoulder harness and load lifter straps throughout the day to alternate the load between your shoulders and hips. I know all frame packs should work this way but when the torso length is just right it makes the load feel much lighter and more balanced. For heavy loads I sometimes shorten up the torso length a bit to keep the hipbelt riding where I want it. The flexible and thin frame also helps with this as the load is closer to your back and doesn't swing when you loosen up the load lifter straps. As a bonus you can strip a lot of weight off the pack to lighten it up. I took off the top pocket and straps, ski carry loops, sleeping pad straps, shortened the compression straps, snipped off a few other things and got it down to 3 pounds. All this should apply to the Aether 70 although it would be heavier.