USB Drive with built-in rsync

When someone wants to back up their computer in a crude fashion, they might simply copy a large folder full of files to their USB hard drive so they can end up with individually readable files on the destination drive without creating some monstrous disk image requiring third-party tools. (It's hard to beat that simplicity and clarity.) An overwrite of everything is unnecessary because most files haven't changed, but if a small piece of a large file HAS changed, standard copy utilities end up copying each whole, changed file, regardless whether only one byte out of a gigabyte changed. (Some good examples include Outlook PST files and VM images.) Proposed is a USB hard drive capable of hashing its contents, communicating just the hashes over the USB channel to save bandwidth, much as rsync does over a network link. In a well-baked implementation, the OS would then be configured to transparently compare hashes in the background to perform implicit synchronization when overwriting existing blocks.