We had been using our WD external HD My Book 1TB successfully until we changed over from a PC to Mac. The problem started when we used the drive for Time Machine backup purposes. Although the Mac recognises it and we can transfer files to it, when we plug it into our LG stereo through the USB port, the drive is no longer recognised and nothing happens.
If the drive is still capable of being seen and accessed from your Mac, then you will need to check with LG about this. I know WD won’t support LG. However, if now you can’t access the drive at all then it’s entirely possible that the process of moving from pc to mac to LG may have corrupted the partition. If that’s the case, check online for some partition recovery software and see if that helps you - that’s if you need the data. Otherwise, you can just repartition and reformat the drive with your Mac.