How can they maintain this if it's free?
This is from their FAQ:
1000 TB? how is this possible?!?! and for free?!
We have seen many posts on different forums claiming that "it is not possible", But it IS possible, and we do it, so we would like to explain how.
First of all, we are dealing with virtual storage and we wrote the filesystem for it. Files will be located on you virtual hard drive and it will look like one physical hard drive, but on our servers they are spread on many many hard drives in RAID arrays on different servers.
Each request for any file is being redirected by our servers, so actuality, files inside one of your folders in the virtual hard drive can be on different servers!
Secondly our servers are smart enough to make only one copy for every file, yet you can edit and change files that you receive from others without affecting other users.
Thirdly all other hosting companies offer unlimited storage, is this possible?
So how do we make money? From premium users of course, like any other hosting company...
Source:
http://www.petandrive.com/help.php