diff -ur calamares-3.2.7/src/modules/unpackfs/main.py calamares-3.2.7-unpackfs-dev/src/modules/unpackfs/main.py
--- calamares-3.2.7/src/modules/unpackfs/main.py	2019-04-27 20:12:03.000000000 +0200
+++ calamares-3.2.7-unpackfs-dev/src/modules/unpackfs/main.py	2019-05-06 00:00:55.302325577 +0200
@@ -252,13 +252,19 @@
             subprocess.check_call(["mount",
                                    "--bind", entry.source,
                                    imgmountdir])
-        else:
+        else if os.path.isfile(entry.source):
             subprocess.check_call(["mount",
                                    entry.source,
                                    imgmountdir,
                                    "-t", entry.sourcefs,
                                    "-o", "loop"
                                    ])
+        else: # entry.source is a device
+            subprocess.check_call(["mount",
+                                   entry.source,
+                                   imgmountdir,
+                                   "-t", entry.sourcefs
+                                   ])
 
     def unpack_image(self, entry, imgmountdir):
         """