I use Dropbox but also like to back up my files to an external hard drive. I am currently doing a one way sync from Dropbox to the external drive every hour. This is very inefficient as the application rechecks all the checksums each time. However, I do this as I don't understand what Dropbox does if you copy a large file to it and then half way through the copying try to copy the file to a backup.
In my set up, I copy the files to my Dropbox folder in location A, and then copy the files from the Dropbox folder in location B to the external hard drive. So Dropbox is basically used as a way to transfer the files to nearer the external hard drive.
Is there a safe and guaranteed way of backing up Dropbox files that doesn't suffer from this sort of possible race condition?
Further clarification: The question is if Dropbox updates the local copies atomically (using locks or renaming or some other trick) and preserves file attributes. However, I believe (now that someone told me) that attributes such as modified time, created time etc. are not carried over between machines.
To be clear, this is not a question about uploading or how Dropbox handles updates on its servers. This is only a question about downloading and whether there is any time period when the local copy of a file is incomplete or inconsistent. The only sensible way I can see to avoid inconsistency would be if a copy is made of a local file that is being modified, the copy is changed, the old copy deleted and the new copy renamed. Is that actually what happens? If the file is very large this has obvious space problems so I don't know if they do it.