Scheduling a daily back up frequency of my internal and external drives.

That isnt possible or how it works I'm afraid. 

Dropbox Backup doesn't back up whole drives like other applications does - it moves certain directories to its own folder and backs them up almost continuously (leaving symlinks in the initial location). 

Dropbox sync (there separate) also doesnt backup drives but rather only things inside \Dropbox and on a continuous real time basis. 

Dropbox say, "Once you plug in your external hard drive and agree to back it up, your files will get backed up to Dropbox Backup". They also claim, "your backup will update automatically every hour while your external hard drive is plugged in". 

Hmm.... is that really happening, or not?

Apologies the above applies to internal drives such as C:\

You can back up full external drives as it says above. However, that is external only, not internal.