Don't copy key fragment to drive if it already contains a full keyring

src/freedombone-keydrive

@@ -127,6 +127,15 @@ if [[ $MASTER_DRIVE == "yes" || $MASTER_DRIVE == "y" || $MASTER_DRIVE == "1" ]];
   exit 0
 fi
 
+# Don't use the USB drive if it already contains a full keyring
+if [ -d $USB_MOUNT/.gnupg ]; then
+  echo 'A full GPG keyring already exists on the USB drive.'
+  echo 'Either reformat the USB drive or use a different drive.'
+  umount -f $USB_MOUNT
+  rm -rf $USB_MOUNT
+  exit 3392
+fi
+
 # Append the username as a subdirectory.
 # This has a down side in that it does identify a given fragment
 # as belonging to a given user, but has the convenience upside