iMac not seeing hard drive

I have a WD external drive which is used to stream my music, films and photos to my TV with My Bookworld. My hard drive is connected to my home hub, however for some reason now cannot be seen on my iMac. Can anyone suggest what might be wrong and how to rectify/troubleshoot the problem 

Hi, to see if I understand correctly, you are connecting an external USB hard drive to the My Book World, and you are accessing this hard drive from your TV? 

The content of the hard drive can be played on my TV, ie movies, music and photos

Hi again, you can try connecting the hard drive directly to your Mac and running a test with Disk Utility. Check the link below for more info. 

How to test a drive for defects or problems on a Mac