Assumption fail. Volume is not necessarily a limiting constraint. Hard drives are dense.
According to this http://wiki.answers.com/Q/How_much_gravel_can_a_dump_truck_hold a dump truck can typically hold 13T. I've googled the weight of a 3.5" hard drive and I get values between 400g and over 1kg. So I weighed an old one, and got a bit over 400g. The 400g drives are those with fewer platters, but I'll go with it for simplicity.
13,000kg / 0.4kg gives you 32,500 hard drives. Continuing with your calculations, 3TB*32,500 = 97,500TB = 780,000Tb. (According to 'man units', hard drive manufacturers use SI units.) 780,000Tb/26h = 780,000Tb/93,600s = 8.333Tb/s or 8333Gb/s.
You'll get 23,200 cubic feet of storage out of a Lockheed C-5 Galaxy cargo plane which is about 32 dumptrucks.
23,200*(12^3)/14 * 0.4kg ~= 1,145,416kg The plane can hold a maximum of 81,600 kg. And fuel economy is horrible.
Not that I disagree with your general point.