Avoid relpath bug on Windows (#16164)

src/pytorch_lightning/utilities/migration/utils.py

@@ -21,6 +21,7 @@
 from lightning_utilities.core.rank_zero import rank_zero_warn
 
 import pytorch_lightning as pl
+from lightning_fabric.utilities.imports import _IS_WINDOWS
 from lightning_fabric.utilities.types import _PATH
 from lightning_fabric.utilities.warnings import PossibleUserWarning
 from pytorch_lightning.utilities.migration.migration import _migration_index
@@ -111,7 +112,10 @@ def _pl_migrate_checkpoint(checkpoint: _CHECKPOINT, checkpoint_path: Optional[_P
 
     # include the full upgrade command, including the path to the loaded file in the error message,
     # so user can copy-paste and run if they want
-    path_hint = os.path.relpath(checkpoint_path, os.getcwd())
+    if not _IS_WINDOWS:  # side-step bug: ValueError: path is on mount 'C:', start on mount 'D:'
+        path_hint = os.path.relpath(checkpoint_path, os.getcwd())
+    else:
+        path_hint = os.path.abspath(checkpoint_path)
     _log.info(
         f"Lightning automatically upgraded your loaded checkpoint from v{old_version} to v{new_version}."
         " To apply the upgrade to your files permanently, run"