BUG: Fix bug with parent dir check

doc/changes/devel.rst

@@ -45,6 +45,7 @@ Bugs
 - Allow :func:`mne.viz.plot_compare_evokeds` to plot eyetracking channels, and improve error handling (:gh:`12190` by `Scott Huberty`_)
 - Fix bug with accessing the last data sample using ``raw[:, -1]`` where an empty array was returned (:gh:`12248` by `Eric Larson`_)
 - Remove incorrect type hints in :func:`mne.io.read_raw_neuralynx` (:gh:`12236` by `Richard Höchenberger`_)
+- Fix bug where parent directory existence was not checked properly in :meth:`mne.io.Raw.save` (:gh:`12282` by `Eric Larson`_)
 - ``defusedxml`` is now an optional (rather than required) dependency and needed when reading EGI-MFF data, NEDF data, and BrainVision montages (:gh:`12264` by `Eric Larson`_)
 
 API changes

mne/io/base.py

@@ -2563,6 +2563,13 @@ def set_annotations(self, annotations):
 def _write_raw(raw_fid_writer, fpath, split_naming, overwrite):
     """Write raw file with splitting."""
     dir_path = fpath.parent
+    _check_fname(
+        dir_path,
+        overwrite="read",
+        must_exist=True,
+        name="parent directory",
+        need_dir=True,
+    )
     # We have to create one extra filename here to make the for loop below happy,
     # but it will raise an error if it actually gets used
     split_fnames = _make_split_fnames(

mne/io/fiff/tests/test_raw_fiff.py

@@ -771,6 +771,10 @@ def test_io_raw(tmp_path):
     sl = slice(inds[0], inds[1])
     assert_allclose(data[:, sl], raw[:, sl][0], rtol=1e-6, atol=1e-20)
 
+    # missing dir raises informative error
+    with pytest.raises(FileNotFoundError, match="parent directory does not exist"):
+        raw.save(tmp_path / "foo" / "test_raw.fif", split_size="1MB")
+
 
 @pytest.mark.parametrize(
     "fname_in, fname_out",