The Mojave cannot recognize external hard drive.

well, i had the same experience one day, when my external drives would not show up... via usb cable.. So i looked online and did all that malarky, finder - preferences, sidebar, etc..., app-disk utility-mount...etc.... ( And these elaborate re-boot and check everything strategies almost never solve any Mac problem i've ever had )

Then today i hovered the mouse in the sidebar of the finder window, just to the right of iCloud, and noticed that "hide" would appear with the mouse hover, and of course if you click "hide" the iCloud drive goes away and the hidden link now says "show"... it toggles.... Ok, so just below that, where Locations would be showing my external drives and stuff, the toggled INVISIBLE link was of course hiding everything, so i hovered there, clicked "show" and Voila ! all my externals were back again...

I don't know what made Apple create these INVISIBLE toggles, or what made my computer toggle it off in the first place, maybe a stray mouse click on my part... Anyway, try this, see if it solves the missing externals for you.