Any drop or knock can cause a head crash in your external hard drive, which means the read/write head of the external hard drive came in contact with the platters. If a hard drive is dropped and it isn’t working anymore, it is likely that the platters, arm or render head of the hard drive were damaged or misaligned from the impact. If the render head is trying to locate the data that it is searching for on the platters and it cannot find it, the arm will move back and forth across the platters several times. The head is attached to an arm that hits a stop every time it goes back and forth, which makes the clicking noise. You might be able to try a few software programs but they won't help much besides logical issues. Software really only addresses issues with reformatting, but even then, resorting to software may make matters worse. Your best bet is that if you truly have stuff of value on there is to send it to professionals. Otherwise, I have heard of a few companies that would send you a replacement but that obviously won't save your data.