--- pathlib.py	2020-04-22 01:25:20.763546402 +0200
+++ pathlib_new.py	2020-04-22 01:34:10.285543587 +0200
@@ -892,14 +892,20 @@
             to_abs_parts = [to_drv, to_root] + to_parts[1:]
         else:
             to_abs_parts = to_parts
-        n = len(to_abs_parts)
+        
         cf = self._flavour.casefold_parts
-        if (root or drv) if n == 0 else cf(abs_parts[:n]) != cf(to_abs_parts):
+        n = 0
+        for p1, p2 in zip(cf(abs_parts), cf(to_abs_parts)):
+            if p1 != p2: break
+            n += 1
+        
+        if n == 0:
             formatted = self._format_parsed_parts(to_drv, to_root, to_parts)
-            raise ValueError("{!r} does not start with {!r}"
-                             .format(str(self), str(formatted)))
+            raise ValueError("{!r} doesn't have any common parents with {!r}"
+                                .format(str(self), str(formatted)))
         return self._from_parsed_parts('', root if n == 1 else '',
-                                       abs_parts[n:])
+                                        (len(to_abs_parts)-n)*['..'] +
+                                        abs_parts[n:])
 
     @property
     def parts(self):