No, Dropbox, will just resync the files. You may need to reinstall the Dropbox app after the install but once Dropbox is back online it will just download those newer files.
And yes putting both backups on the same media creates a risk (primarily of some problem that would destroy your primary and backup drives at the same time). If just the backup failed, you'd lose the backups but could always buy another drive and run two more backups. That said, the prudent approach is maintain 2 separate backups on separate physical disks. Also, having a 3rd disk that can be taken to another home or safe deposit box will mitigate fire, or other damage that might take out everything in your home at once.
You could keep going and try to make sure you never, ever lose anything but it takes time and money the more redundancy you create.