Why does Dropbox upload files when I move them?

Question

In Windows File Manager, with files that are not online-only -- when I move a set of files from one Dropbox folder to another, it often shows that these files are being uploaded from my PC to Dropbox all over again.  I'm not positive that this happens every time, but it seems very inefficient.  Is it supposed to do this?

Rich · Accepted Answer

@Karl H.4 wrote:In Windows File Manager, with files that are not online-only -- when I move a set of files from one Dropbox folder to another, it often shows that these files are being uploaded from my PC to Dropbox all over again.

It may say that it's uploading files, but it's smart enough to know that it doesn't need to.

When you move or rename a file, Dropbox doesn't see that as a move, but as a deletion and a creation. Dropbox is notified by your operating system that a file was removed in one location and a new file appeared in a different location. Dropbox then indexes the "new" file, which consists of splitting the file into 4MB chunks and then hashing each chunk. Those hashes are then sent to the Dropbox server and compared to the hashes that they already have for your account. If it finds a matching hash, it knows that it doesn't need to upload that particular chunk of the file and moves on to the next.

What you may see as uploading of files is likely just the hashes, or comparison data, for each file. The only files that actually need to transfer are those that Dropbox doesn't have record of.

There's a pretty easy way to see how this happens as well. Upload a rather large file to Dropbox. It doesn't have to be too big; just something that uploads in a time that you can measure. Once the file is fully synced and it has a green check, make a copy of it in another folder elsewhere in your account, then time how long it takes to upload that file. It should finish MUCH faster, usually in a matter of seconds, because Dropbox knows the file already exists in your account.