Should I spread data across multiple drives?

None of that works if your house catches fire though... If it's importance you should have an off site backup also.

Yep, something like Dropbox isn't really for archiving or backing up. One thing to remember is that the normal way of working is to sync with folders on your computer. If you delete a file accidently you delete it from Dropbox too.

You can bring back deleted files for a while but you wouldn't want to rely on it