With it plugged in, open up Disk Utility. Make sure that it is registering the device. Select it and click on 'First Aid'. It will check to see if there's a problem with the drive. If it does not give you any errors, click 'Mount'. If it's an unsupported partition, you'll have to 'partition/erase' it so you can use it(NOTE: you will lose everything on the drive once you do this).