The service hosted by google is supported by GFS, the google filesystem.
I was not sure until I spot this post from whom seems to be a google employee.
Edit:
The fact that Google use GFS is actually an inference based on the fact that the filesystem fits perfectly the model of google drive. An article has been published on arstechnica, which describe that in more details.
Of course no one expect former and current google employees knows what is behind, but the fact that someone who claims being one of them specifically points the OP of the question I linked to GFS, is another strong incentive to believe that GFS (and maybe BigTable) are backing up Google Drive.
Edit 2:
As I was seeing downvotes and that the topic actually interest me a lot, I decided to enrich this answer with the following arguments:
1) Google infrastructure strategy is to build clusters of inexpensive commodity hardware, with in mind the fact that it will fail. This was one of the motivation behind he GFS infrastructure.
2) The intuition that, as GFS totally fits Google's infrastructures, services and internal softwares (among which MapReduce is an important actor), has lead other people to the conclusion that it is backing up pretty much all their services. See :
http://www.slideshare.net/hasanveldstra/the-anatomy-of-the-google-architecture-fina-lv11.
Also, an interview of Jeff Dean, backup a lot this intuition, and explain 1) better
3) That doesn't have any actual meaning, but I found it fun: some user actually ended up having the extension .gfs
in Drive (It'd be unexpected, but somehow unfortunate that this gfs actually refer to MS Groove files)
I don' think we'll ever see an actual former/current employee validate this statement, and of course much of the details of what is backing up Drive will stay hidden, but the intuition has strong roots.