path: local: str return full path on value error

dvc/path/local.py

@@ -8,4 +8,10 @@ class PathLOCAL(PathBASE):
     scheme = Schemes.LOCAL
 
     def __str__(self):
+        if os.name == "nt" and not os.path.commonprefix(
+            [os.getcwd(), self.path]
+        ):
+            # In case of windows, when cache is on different drive than
+            # workspace, we will get ValueError when trying to get relpath
+            return self.path
         return os.path.relpath(self.path)

tests/conftest.py

@@ -97,3 +97,36 @@ def ssh(ssh_server):
         port=ssh_server.port,
         key_filename=key_path,
     )
+
+
+@pytest.fixture
+def temporary_windows_drive(repo_dir):
+    import string
+    import win32api
+    from ctypes import windll
+    from win32con import DDD_REMOVE_DEFINITION
+
+    drives = [
+        s[0].upper()
+        for s in win32api.GetLogicalDriveStrings().split("\000")
+        if len(s) > 0
+    ]
+
+    new_drive_name = [
+        letter for letter in string.ascii_uppercase if letter not in drives
+    ][0]
+    new_drive = "{}:".format(new_drive_name)
+
+    target_path = repo_dir.mkdtemp()
+
+    set_up_result = windll.kernel32.DefineDosDeviceW(0, new_drive, target_path)
+    if set_up_result == 0:
+        raise RuntimeError("Failed to mount windows drive!")
+
+    yield new_drive
+
+    tear_down_result = windll.kernel32.DefineDosDeviceW(
+        DDD_REMOVE_DEFINITION, new_drive, target_path
+    )
+    if tear_down_result == 0:
+        raise RuntimeError("Could not unmount windows drive!")

tests/func/test_add.py

@@ -2,6 +2,8 @@ from __future__ import unicode_literals
 
 import os
 
+import pytest
+
 import dvc
 import time
 import shutil
@@ -541,3 +543,14 @@ class TestAddUnprotected(TestDvc):
 
         self.assertFalse(os.access(self.FOO, os.W_OK))
         self.assertTrue(System.is_hardlink(self.FOO))
+
+
+@pytest.mark.skipif(os.name != "nt", reason="Windows specific")
+def test_windows_should_add_when_cache_on_different_drive(
+    dvc, repo_dir, temporary_windows_drive
+):
+    ret = main(["config", "cache.dir", temporary_windows_drive])
+    assert ret == 0
+
+    ret = main(["add", repo_dir.DATA])
+    assert ret == 0