WD My Book not showing in finder when plugged into USB

So I have a 2TB WD My Book that I use as a time machine backup. I currently have it connected to an AirtPort Extreme Base Station. When connected this way I can access the drive from Finder with no issue.

However, when I connect it to a USB on the back of my iMac, the following symptoms will happen

1. The drive does not mount/ cannot be found in Finder
2. Disk Utility does not load any of the drives, just spins

   Disk Utility will load if I disconnect the external drive.

3. The drive is recognized by the WD Drive Utilities app

The reason I am trying to connect it directly via USB is because finder says it is full, when I know for a fact it is not. So I wanted to run the Disk Utilities on it. Also am trying to figure out what, if anything, is taking up so much space.