macOS - External drives doesn't show up in Finder External hard disks and pen drives connected via USB no longer show up in Finder.

What could be the possible reason, and how do I resolve this issue?
I have had this issue, on my 2017 MacBook Pro (2 USB-C ports) - and I found that the problem is resolved by resetting the Mac's System Management Controller (SMC).

Power your machine down, wait a couple of seconds - then press the left-hand side Shift-Control-Option at the same time, as well as the power button, for 10 seconds (I count slowly, so more likely 15 seconds when I do it). Release the keys and then power your machine back on.

Hopefully it will now register the external drives.