These storage pods (http://www.45drives.com/products/direct-wired-standard.php http://www.45drives.com/products/direct-wired-standard.php) hold 180TB if you pop

These storage pods (http://www.45drives.com/products/direct-wired-standard.php http://www.45drives.com/products/direct-wired-standard.php) hold 180TB if you populate them with 4TB drives. 20 PB / 180 TB ~= 111. With some redundancy, you could round up to 200 pods. Now you host these in various datacentres around the world. Maybe one rack per data centre so approx 20 datacentres. This will be manageable, sort of like managing a small cluster. You'll have plenty of cheap bandwidth and excellent reliability/availability. Datacentres may be able to donate space or bandwidth to the project for the positive publicity, etc. Relying on USB drives hanging off of consumer's laptops connected over wifi is going to have dismal reliability/availability, poor bandwidth, etc, so to compensate you'll need a colossal amount of redundancy (5-10x? more?) for it to even function. From an end user's point of view, I think my donation of $100 toward the above storage pod approach would go a lot further than me buying a $100 USB drive and managing a low quality bandwidth capped contribution.