Are you pressing the Eject button on the remote before pulling out the usb cable from the WDTV? You need to press eject, WDTV will then ask you which drive you want to eject, select the one you want, press OK. A green tick will appear, you need to press OK again, now it it safe to pull out the usb cable. If you don’t do this then it won’t read when you plug it back in.
Same thing on your PC, you can’t just pull out the cable. You need to use the “Safely Remove Hardware” function on the PC. When the PC tells you it’s safe to remove, then you can pull out the USB cable.
Try this if it’s not working - plug it back into your PC. Run a CHKDSK. You might get a message saying that you don’t need to scan the drive as there are no errors, but scan it anyway. If it asks you if you want a quick scan or a full scan choose the quick one. When it’s finished use the “Safely Remove Hardware” function on the PC before you pull out the cable.