If that's all you want, I'm sure you can find a Windows equivalent to dd to run every day. Do a direct bit copy, and then your external disk will always be in sync with your internal disk.
Arq is a good backup-to-cloud service if you're conscious of where your data goes(S3) and who has the keys(Only you).
They're both indie programmers out of Boston too, which may be some of my bias, but check'm out!