diff --git a/src/virtualship/cli/_run.py b/src/virtualship/cli/_run.py
index 2feb43a3..f2622be3 100644
--- a/src/virtualship/cli/_run.py
+++ b/src/virtualship/cli/_run.py
@@ -255,36 +255,35 @@ def _unique_id(expedition: Expedition, cache_dir: Path) -> str:
     """
     Return a unique id for the expedition (marked by datetime), which can be used to determine whether the expedition has 'changed' since the last run.
 
-    Simultaneously, log to .txt file if first run or if there have been additions of instruments since last run.
+    Returns the previous id if no instruments have been added since the last run, allowing re-use of previously encountered problems.
+    Otherwise generates and persists a new id.
     """
-    current_instruments = expedition.get_instruments()
-    new_id = datetime.now().strftime("%Y%m%d%H%M%S")
-    previous_id = None
+    cache_dir.mkdir(exist_ok=True)
 
-    if not cache_dir.exists():
-        cache_dir.mkdir()
     id_path = cache_dir.joinpath(EXPEDITION_IDENTIFIER)
     last_expedition_path = cache_dir.joinpath(EXPEDITION_LATEST)
+    new_id = datetime.now().strftime("%Y%m%d%H%M%S")
 
-    if id_path.exists():
-        previous_id = id_path.read_text().strip()
-        try:
-            last_expedition = Expedition.from_yaml(last_expedition_path)
-        except FileNotFoundError as e:
-            raise RuntimeError(
-                f"Previous expedition data is present but incomplete in {cache_dir}. This may be because a previous expedition run was interrupted/failed unexpectedly. Deleting the '{cache_dir}' directory and re-running the expedition should resolve this issue."
-            ) from e
-        last_instruments = last_expedition.get_instruments()
+    if not id_path.exists():
+        id_path.write_text(new_id)
+        return new_id
 
-        added_instruments = set(current_instruments) - set(last_instruments)
-        if not added_instruments:
-            return previous_id  # if no additions, keep previous id to allow re-use of previously encountered problems
-        else:
-            id_path.write_text(new_id)
+    previous_id = id_path.read_text().strip()
 
-    else:
+    try:
+        last_expedition = Expedition.from_yaml(last_expedition_path)
+    except FileNotFoundError:
+        # cache is not useful in this case as it implies the previous run was interrupted and is incomplete; update passively
         id_path.write_text(new_id)
+        return new_id
+
+    added_instruments = set(expedition.get_instruments()) - set(
+        last_expedition.get_instruments()
+    )
+    if not added_instruments:
+        return previous_id  # if no additions, keep previous id to allow re-use of previously encountered problems
 
+    id_path.write_text(new_id)
     return new_id
 
 
diff --git a/tests/cli/test_run.py b/tests/cli/test_run.py
index bbdc2fa9..b537e405 100644
--- a/tests/cli/test_run.py
+++ b/tests/cli/test_run.py
@@ -1,12 +1,14 @@
 from datetime import datetime
 from pathlib import Path
+from unittest.mock import MagicMock
 
-from virtualship.cli._run import _run
+from virtualship.cli._run import _run, _unique_id
 from virtualship.expedition.simulate_schedule import (
     MeasurementsToSimulate,
     ScheduleOk,
 )
-from virtualship.utils import EXPEDITION, get_example_expedition
+from virtualship.instruments.types import InstrumentType
+from virtualship.utils import EXPEDITION, EXPEDITION_IDENTIFIER, get_example_expedition
 
 
 def _simulate_schedule(projection, expedition):
@@ -71,3 +73,24 @@ def test_run(tmp_path, monkeypatch):
     if difficulty_level == "easy":
         cache_dir = expedition_dir / "cache"
         assert not cache_dir.exists()
+
+
+def test_unique_id_incomplete_cache_does_not_raise(tmp_path):
+    """When expedition_latest.yaml is missing (incomplete cache from an interrupted run), _unique_id must not raise error for any 'difficulty_level', it should return a new id."""
+    ORIGINAL_TIMESTAMP = "20240101120000"
+
+    cache_dir = tmp_path / "cache"
+    cache_dir.mkdir()
+    (cache_dir / EXPEDITION_IDENTIFIER).write_text(ORIGINAL_TIMESTAMP)
+    # EXPEDITION_LATEST intentionally absent to simulate an interrupted run
+
+    expedition = MagicMock()
+    expedition.get_instruments.return_value = {InstrumentType.CTD}
+
+    result = _unique_id(expedition, cache_dir)
+
+    assert result != ORIGINAL_TIMESTAMP
+    assert (
+        abs(datetime.strptime(result, "%Y%m%d%H%M%S") - datetime.now()).seconds < 30
+    ), "new ID should be timestamp close to the current time"
+    assert (cache_dir / EXPEDITION_IDENTIFIER).read_text().strip() == result