External devices not getting detected on your Mac or PC would often happen mostly due to interface issues so, that your WD external hard drive not recognized mac .
If you have connected your WD external hard drive to your Mac using a USB cable, make sure that the cable is not defective. Check if the USB ports on the Mac are damaged. If you find the connected port faulty, connect the cable to another port.
If there seems to be no issue with the USB cable and the USB port and still the WD external hard drive is not recognized by your Mac, consider the procedure given below to fix the issue.
Still, if the WD external hard drive not recognized mac , connect it to some other Mac or PC and check if it is detected. If it is not detected, the drive might be faulty and you should consider replacing it.