backup external drive

Bella appears to be "grayed out" in the screenshot of your list of attached disks to your Mac, likely indicating that it is not formatted correctly in Mac OS Extended (Journaled), which Time Machine requires.

Assuming that Bella is connected directly to your Mac via USB or Thunderbolt, you can check on the compatibility of Bella as follows:

Right-Click on the icon for Bella on the Mac desktop

Click on Get Info

Look for Format in the new window that appears on the Mac's screen

What is the format of the Bella hard drive?

Please provide a screen shot image of the previous window to this one. It will show which drives have been selected for Time Machine backups. I am assuming the Bella would be listed there ... is it?