This isn't an enterprisey backup solution by any means, as I don't believe previous versions are stored locally. But it's a whole lot of really convenient redundancy.
1) Have all your nodes syncing at the same time and connected.
2) Introduce file corruption and deletion.
3) Have all your sync'd machines get the deletion.
4) Have Dropbox's file revision history go wonky on their side.
That's not necessary. All you need is your nodes to connect and sync at a time before you notice a problem, or if after you notice a problem but accidentally let it connect (consider a non-technical user here).
> 2) Introduce file corruption and deletion.
If this didn't happen, you wouldn't need backups. By discussing backups, we're already assuming this might happen. Unless you consider backups to protect only against theft or fire. Accidental deletion and corruption are also major factors.
> 3) Have all your sync'd machines get the deletion.
That's what the system is designed to do, so that's a given.
> 4) Have Dropbox's file revision history go wonky on their side.
There well be one error that leads to both file revision history going wonky and the introduction of file corruption or deletion.
I'm not having a go at Dropbox, it works as expected. But backups need to be independent, not heavily integrated. Otherwise what you get is some kind of pseudo-backup that won't cut it in particular, relatively common failure modes.
It certainly isn't "one heck of a failure". It's one failure.
Is that correct? My understanding is that all your computers are connected to the same remote server on which your files reside, so that they all have access to the latest version. That does nothing to prevent loss of access / loss of data if the remote server goes down.
What that means is, you work with the files as if they were completely local to your computer, since they are. Dropbox monitors for changes, and makes sure to update all the other copies of these files. But you always get the speed (and convenience) of working with local files.
That also means that what the parent says is right - you have extra copies of your files lying around on any computer that is connected to Dropbox. These are full physical copies of the latest version of each file.
Of course, if there is some bug that makes Dropbox think all your files were deleted or something, that change could propagate to all your other computers at once and delete all your backups. I agree that it's not very likely.
Not very likely, and if you've something like OSX's Time Machine it'd be trivial to go back.