You should always have a backup of your important files and documents on an external hard drive because if a virus attacked and wiped out your internal desktop hard drive, then all your important files and documents will be lost forever. it is a very painful and heart broken feeling to have when something like that happens.
You should only attach your external hard drive onto your desktop when you are doing a backup of files and documents onto the external hard drive. After completing the backup, you should always remember to disconnect your external hard drive and do not leave it plugged onto the desktop. This is to prevent the virus if in case, from attacking your external hard drive as well, and wiping the contents of your external hard drive. Besides, these days the price an external hard drive is quite cheap compared to years before. Also the storage space of the external hard drive had increased a lot and the physical size of the external hard drive had thankfully reduced to compact size.
I remember when external hard drive just started becoming popular, at that time we had to plug one end of the external hard drive onto the power outlet and then plug the USB connection from the external hard drive to the USB port of the Desktop or Laptop. Nowadays, we don't need to do that. The power is supported simply just by plugging the USB from the external hard drive into the USB port of desktop or laptop.
Check out the different types of external hard drives and the company that makes them here:
Portable external hard drive USB storage