Another thing to try ... first, be in the Finder (main screen of your Mac). See the two screenshots I'll attach here. Go up to "File" (upper left), click, then select 'Preferences...". When a small window opens, make sure you check the boxes for the various kinds of drives, so that all drives show up on the desktop. Sometimes one or several are not checked; put a checkbox in them. This should make the USB drive(s) show up on your desktop.