As long as you can do su, there's actually no need to log in as root! In fact, this is most definitely a safety feature (not being able to login as root, I mean), many people change their system's behaviour to do exactly that.
Solutions without changing the safety level:
- If you can, log in with your user and use su - it's more healthy that way.
- Install and use sudo, adding your user to /etc/sudoers - but for safety's sake, have sudo use a password. man sudo and man sudoers are very helpful.
- Later on, you may even want to write a simple script to do your backup tasks for you (that can be run by cron as root).