Many Web services that offer a virtual hard drive in the cloud have come under scrutiny for their privacy policies, and you should understand why. Unlike storing all your files on your own computer or a physical external hard drive, with a virtual hard drive, you're uploading your files to a server. This means that, technically, you're not the only one with access to them.
How each service states that borders on elegant. Google Drive, for instance, gives a long list of things it has permission to do with your content. Microsoft somewhat sternly notes that, for instance, if you're infringing on copyrights, you're not following the rules and can be kicked off. Dropbox just says you're agreeing to "the permissions we need" to run its services [source: Patel]. Apple's iCloud comes straight out and says it can remove "objectionable" material -- whatever that may be, as the company also gives itself permission to "determine whether Content is appropriate" [source: Patel].
So is your content being monitored? Well, maybe. The beauty of the privacy policies is that you can kind of assume your stuff is being looked at. For instance, how would Microsoft know you're infringing on a copyright or Apple realize your material is "objectionable" if someone wasn't looking at it? Then again, Google implies that -- just like when it scans your e-mail -- it's simply helping along marketing by scanning for words and targeting ads to you. But the company basically has the permissions to do whatever it wants with your content: Google can "use, host, store, reproduce, modify, create derivative works (such as those resulting from translations, adaptations or other changes we make so that your content works better with our Services), communicate, publish, publicly perform, publicly display and distribute such content" as long as it's for "operating, promoting, and improving our Services, and to develop new ones" [source: Patel].
So while a virtual hard drive might be a terrific option for storage and sharing, do be aware that we're not entirely sure what the Web service is seeing.