I think #1 covers it. It feels a bit awkward to even refer to file-level hashing and lookup as de-duplication as it's more commonly known in block-level de-dup

I think #1 covers it. It feels a bit awkward to even refer to file-level hashing and lookup as de-duplication as it's more commonly known in block-level de-dupe. Regardless, on documents it's probably safe to assume they get close to 2:1 compression ratio. I'd assume that the de-dupe of large binaries is a huge part of what makes running such a business on very expensive cloud-storage systems financially viable. Would Dropbox work financially without it? They could offer a non-de-duped version at much higher cost for the security minded, but then they've implied that the basic version is fundamentally flawed. Just an interesting technical barriers to think about (IMO).