diff --git a/src/TDB2.cpp b/src/TDB2.cpp
index b5f146452..b729c9a63 100644
--- a/src/TDB2.cpp
+++ b/src/TDB2.cpp
@@ -57,6 +57,14 @@ TDB2::TDB2 ()
 ////////////////////////////////////////////////////////////////////////////////
 void TDB2::open_replica (const std::string& location, bool create_if_missing)
 {
+  File pending_data = File (location + "/pending.data");
+  if (pending_data.exists()) {
+    Color warning = Color (Context::getContext ().config.get ("color.warning"));
+    std::cerr << warning.colorize (
+      format ("Found existing '.data' files in {1}", location)) << "\n";
+    std::cerr << "  Taskwarrior's storage format changed in 3.0, requiring a manual migration.\n";
+    std::cerr << "  See https://github.com/GothenburgBitFactory/taskwarrior/releases.\n";
+  }
   replica = tc::Replica(location, create_if_missing);
 }