Setting Up Backup Storage Question

I just purchased a My Cloud EX2 Ultra and set it up with 2 8tb drives in Raid 1, I am using it as a shared drive to store my movies and want to add an external drive for backups and my questions are:

  1. Would an 8tb external drive be large enough for backups and what is the general rule of thumb for backup drive sizes?

  2. Isn’t it risky to leave the Backup drive plugged in all the time, since it will be just as vulnerable to viruses/malware and failure as the main drive?

  3. What would my best options be if I wanted it to backup my content every 24 hours without having to unplug and reconnect before every backup?

I’d like to avoid using a cloud service to backup the content, since I feel that it kind of defeats the point of having the My Cloud and don’t want the monthly fees

External drive with My Cloud can be used to create a backup of My Cloud into it and need to start a backup job manually from the dashboard. You can connect external drive with My Cloud only when you want to take a backup, as it would be done manually.

Backup drive should be some bit larger than the data drive as backup software used to store image of files which would be updated at next scheduled backup process. WD Smartware is the supported backup tool provided by WD with scheduled backup feature.