Unlike a system that stores your cumulative history, Backblaze simply keeps copies of every file you have on you computer right now. If you delete something from your local drive, Blackblaze notices, and flags their copy for deletion in 30 days. In other words, they're in the data mirroring and recovery business, not the general archive trade (a point they make very clear in their promotional materials).

They also protect themselves from overload by limiting the drives they backup to local ones (internal or external), and explicitly excluding network drives. In other words, while the amount of data per machine is unlimited, the number of machines per account is limited very strictly. This distinction is what keeps them viable.

There is an additional limit on what they handle imposed by network speeds. Realistically, most people can't send more than 4-5 GB per day. For some, it's as low as 2-3, and if you can deliver 9-10, you're doing well. Blackblaze notes this clearly, pointing out that an initial backup can take weeks, if not months to complete.

What makes them especially awesome is that they don't expect you to wait this long to recover your data. You can download files via their website at no charge. Alternately, you can flag larger collections, have them copied to DVD, and FedEx'd for next-day delivery. And if you've got a lot of stuff you need immediately, they can put up to 400 GB on a single USB drive and overnight that to you for $250.

Obviously, their proposition doesn't serve everyone. But for a certain class of customers, it's absolutely perfect.

So much for the "damn, I must have deleted that file; I guess I'll have to recover it from my backups" scenario.