diff --git a/scripts/slot.py b/scripts/slot.py
index f08049ad..f79c11ce 100644
--- a/scripts/slot.py
+++ b/scripts/slot.py
@@ -23,6 +23,8 @@ The scale is only meaningful inside one emulator's candidate set. When two
 profiles rank the same file differently -- pcsx1 walks scph1001 first while
 DuckStation prefers scph5501 -- the pack serves both cores, so neither order
 may win: the file is marked conflicting and its group stays undecidable.
+The same holds when one profile ranks a file and another declares it with no
+rank: the second core reads it by its own rule, which no rank expresses.
 """
 
 from __future__ import annotations
@@ -62,10 +64,11 @@ def build_slot_index(profiles: dict) -> dict[str, dict]:
                 entry = index.setdefault(
                     key,
                     {"rank": None, "conflict": False, "regions": set(),
-                     "emulators": []},
+                     "emulators": [], "ranked_by": set(), "unranked_by": set()},
                 )
                 entry["regions"] |= {str(r) for r in (f.get("region") or [])}
                 rank = f.get("priority")
+                entry["unranked_by" if rank is None else "ranked_by"].add(emu_name)
                 if rank is not None:
                     current = entry["rank"]
                     if current is not None and current != rank:
@@ -75,6 +78,14 @@ def build_slot_index(profiles: dict) -> dict[str, dict]:
                     )
                 if emu_name not in entry["emulators"]:
                     entry["emulators"].append(emu_name)
+    # A core that declares the file without a rank loads it by a rule of its
+    # own: Genesis Plus GX opens bios_CD_U.bin by name for a US disc, and
+    # PicoDrive's rank 4 for it would let its rank-3 file replace it.
+    for entry in index.values():
+        if entry["ranked_by"] and entry.pop("unranked_by") - entry["ranked_by"]:
+            entry["conflict"] = True
+        entry.pop("unranked_by", None)
+        entry.pop("ranked_by")
     return index
 
 
diff --git a/tests/test_slot.py b/tests/test_slot.py
index 9cd29878..402c2cdb 100644
--- a/tests/test_slot.py
+++ b/tests/test_slot.py
@@ -260,5 +260,42 @@ class TestRepoProfiles(unittest.TestCase):
         self.assertEqual(undecidable, ["sony-playstation|japan"])
 
 
+
+class AnUnrankedDeclarationIsADisagreement(unittest.TestCase):
+    """ROCKNIX --one-per-slot dropped the required bios_CD_U.bin: PicoDrive
+    ranks it 4 behind its own us_scd1_9210.bin, while Genesis Plus GX opens
+    bios_CD_U.bin by name and ranks nothing."""
+
+    def test_the_file_genesis_plus_gx_opens_is_kept(self):
+        from slot import build_slot_index, resolve_slot_drops
+
+        profiles = {
+            "picodrive": {"files": [
+                {"name": "us_scd1_9210.bin", "priority": 3},
+                {"name": "bios_CD_U.bin", "priority": 4},
+            ]},
+            "genesis_plus_gx": {"files": [{"name": "bios_CD_U.bin"}]},
+        }
+        index = build_slot_index(profiles)
+        drops, undecidable = resolve_slot_drops(
+            {"sega-mega-cd": [("bios/us_scd1_9210.bin", "us_scd1_9210.bin"),
+                              ("bios/bios_CD_U.bin", "bios_CD_U.bin")]},
+            index,
+        )
+        self.assertEqual(drops, set())
+        self.assertEqual(undecidable, ["sega-mega-cd"])
+
+    def test_one_core_ranking_alone_still_decides(self):
+        from slot import build_slot_index, resolve_slot_drops
+
+        index = build_slot_index({"picodrive": {"files": [
+            {"name": "a.bin", "priority": 1}, {"name": "b.bin", "priority": 2},
+        ]}})
+        drops, _ = resolve_slot_drops(
+            {"g": [("bios/a.bin", "a.bin"), ("bios/b.bin", "b.bin")]}, index
+        )
+        self.assertEqual(drops, {"bios/b.bin"})
+
+
 if __name__ == "__main__":
     unittest.main()