Dropbox isn't secure at all!

Are you using Dropbox to securely store/backup your private files?

Think again! Dropbox can determinate if a file uploaded before by other Dropbox user you can find it out "simply by observing the amount of data transferred between your own computer and Dropbox's servers. If the file isn't already stored by Dropbox, the entire file will be uploaded. If Dropbox has the file already, just a few kb of communication will occur."

"What this means, is that from the comfort of their desks, law enforcement agencies or copyright trolls can upload contraband files to Dropbox, watch the amount of bandwidth consumed, and then obtain a court order if the amount of data transferred is smaller than the size of the file"

Source