USB Drives on 2013 iMac missing?

Unplug each USB drive then restart in Safe Mode per the directions below.

Please restart the computer in Safe Mode per the directions in the  How to use safe mode on your Mac - Apple Support link.

Note: If you are using a wireless keyboard please either plug it in using the supplied Lightning cable that shipped with it. If you are using a legacy keyboard or a third-party keyboard that does not have re-charging abilities, please use any USB keyboard. Also note that a Safe Mode restart may require you to hold down the shift key for up to 5 minutes before the computer starts in Safe Mode. You will know it is in Safe Mode when you see “Safe Mode” in the upper right of the display.

Next, please restart normally and please retest.

Go to the Finder Preferences and make sure that External Disks is checked for the Sidebar.