At the time of writing, Dropbox would be a good way to mitigate ransomware attacks because a 30 day version history of file changes is kept on their servers (even on the free tier).

This, depending on the volume of data, requires a fast internet connection for both upload and download for it to be effective.

However, **(big caveat)** it wouldn't take much for new ransomware to be engineered that grabs your session token from Dropbox.com, or that installs a keylogger in order to capture your cloud provider password and then proceeds to select the "Permanently Delete" option, rendering the files irrecoverable.

The same goes for any online storage options, whether mapped as a drive or not, as ransomware could easily be engineered to search out SMB shares on the local network and encrypt files there too. The only real option for online backup would be if you had a write only option where the network protocol will allow new files, changes and deletions only with full version control and no possible way to disable the version control or to permanently delete past copies.

This then leaves offline backups as the only real option. These would have to be manually initiated to removable media, which would be best stored encrypted off-site for protection against non-malware threats (e.g. fire or theft).