macOS - External drives doesn't show up in Finder
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.