diff --git a/src/calibre/devices/scanner.py b/src/calibre/devices/scanner.py
index c7aeeadadf..4088f256be 100644
--- a/src/calibre/devices/scanner.py
+++ b/src/calibre/devices/scanner.py
@@ -21,11 +21,11 @@ if iswindows:
     drive_ok_lock = Lock()
 
     def drive_is_ok(letter, max_tries=10, debug=False):
-        import win32file
+        from calibre_extensions import winutil
         with drive_ok_lock:
             for i in range(max_tries):
                 try:
-                    win32file.GetDiskFreeSpaceEx(letter+':\\')
+                    winutil.get_disk_free_space(letter+':\\')
                     return True
                 except Exception as e:
                     if i >= max_tries - 1 and debug:
diff --git a/src/calibre/devices/usbms/device.py b/src/calibre/devices/usbms/device.py
index 6974a6a701..98cb16104d 100644
--- a/src/calibre/devices/usbms/device.py
+++ b/src/calibre/devices/usbms/device.py
@@ -146,20 +146,17 @@ class Device(DeviceConfig, DevicePlugin):
         if not prefix:
             return 0, 0
         prefix = prefix[:-1]
-        import win32file, winerror
+        from calibre_extensions import winutil
         try:
-            sectors_per_cluster, bytes_per_sector, free_clusters, total_clusters = \
-                win32file.GetDiskFreeSpace(prefix)
-        except Exception as err:
-            if getattr(err, 'args', [None])[0] == winerror.ERROR_NOT_READY:
+            available_space, total_space, free_space = winutil.get_disk_free_space(prefix)
+        except OSError as err:
+            if err.winerror == winutil.ERROR_NOT_READY:
                 # Disk not ready
                 time.sleep(3)
-                sectors_per_cluster, bytes_per_sector, free_clusters, total_clusters = \
-                    win32file.GetDiskFreeSpace(prefix)
+                available_space, total_space, free_space = winutil.get_disk_free_space(prefix)
             else:
                 raise
-        mult = sectors_per_cluster * bytes_per_sector
-        return total_clusters * mult, free_clusters * mult
+        return total_space, available_space
 
     def total_space(self, end_session=True):
         msz = casz = cbsz = 0
diff --git a/src/calibre/utils/windows/winutil.cpp b/src/calibre/utils/windows/winutil.cpp
index 59d5cee249..79e28b101f 100644
--- a/src/calibre/utils/windows/winutil.cpp
+++ b/src/calibre/utils/windows/winutil.cpp
@@ -408,7 +408,11 @@ winutil_get_disk_free_space(PyObject *self, PyObject *args) {
 	wchar_raii path;
 	if (!PyArg_ParseTuple(args, "O&", py_to_wchar, &path)) return NULL;
     ULARGE_INTEGER bytes_available_to_caller, total_bytes, total_free_bytes;
-    if (!GetDiskFreeSpaceEx(path.ptr(), &bytes_available_to_caller, &total_bytes, &total_free_bytes)) return PyErr_SetExcFromWindowsErrWithFilenameObject(PyExc_OSError, 0, PyTuple_GET_ITEM(args, 0));
+	BOOL ok;
+	Py_BEGIN_ALLOW_THREADS
+	ok = GetDiskFreeSpaceEx(path.ptr(), &bytes_available_to_caller, &total_bytes, &total_free_bytes);
+	Py_END_ALLOW_THREADS
+    if (!ok) return PyErr_SetExcFromWindowsErrWithFilenameObject(PyExc_OSError, 0, PyTuple_GET_ITEM(args, 0));
     return Py_BuildValue("KKK", bytes_available_to_caller.QuadPart, total_bytes.QuadPart, total_free_bytes.QuadPart);
 }
 
@@ -1281,6 +1285,8 @@ CALIBRE_MODINIT_FUNC PyInit_winutil(void) {
     PyModule_AddIntConstant(m, "ERROR_MORE_DATA", ERROR_MORE_DATA);
     PyModule_AddIntConstant(m, "ERROR_NO_MORE_ITEMS", ERROR_NO_MORE_ITEMS);
     PyModule_AddIntConstant(m, "ERROR_FILE_NOT_FOUND", ERROR_FILE_NOT_FOUND);
+    PyModule_AddIntConstant(m, "ERROR_GEN_FAILURE ", ERROR_GEN_FAILURE);
+    PyModule_AddIntConstant(m, "ERROR_INSUFFICIENT_BUFFER", ERROR_INSUFFICIENT_BUFFER);
     PyModule_AddIntConstant(m, "ERROR_BAD_COMMAND", ERROR_BAD_COMMAND);
     PyModule_AddIntConstant(m, "ERROR_INVALID_DATA", ERROR_INVALID_DATA);
     PyModule_AddIntConstant(m, "ERROR_NOT_READY", ERROR_NOT_READY);
diff --git a/src/calibre/utils/winreg/default_programs.py b/src/calibre/utils/winreg/default_programs.py
index 26657857d6..f15d0865e2 100644
--- a/src/calibre/utils/winreg/default_programs.py
+++ b/src/calibre/utils/winreg/default_programs.py
@@ -8,13 +8,13 @@ __copyright__ = '2015, Kovid Goyal <kovid at kovidgoyal.net>'
 import os, sys, time, traceback
 from threading import Thread
 
-import winerror
 
 from calibre import guess_type, prints
 from calibre.constants import is64bit, isportable, isfrozen, __version__, DEBUG, plugins
 from calibre.utils.winreg.lib import Key, HKEY_CURRENT_USER, HKEY_LOCAL_MACHINE
 from calibre.utils.lock import singleinstance
 from polyglot.builtins import iteritems, itervalues
+from calibre_extensions import winutil
 
 # See https://msdn.microsoft.com/en-us/library/windows/desktop/cc144154(v=vs.85).aspx
 
@@ -197,7 +197,7 @@ def get_prog_id_map(base, key_path):
     try:
         k = Key(open_at=key_path, root=base)
     except OSError as err:
-        if err.winerror == winerror.ERROR_FILE_NOT_FOUND:
+        if err.winerror == winutil.ERROR_FILE_NOT_FOUND:
             return desc, ans
         raise
     with k:
@@ -213,7 +213,7 @@ def get_open_data(base, prog_id):
     try:
         k = Key(open_at=r'Software\Classes\%s' % prog_id, root=base)
     except OSError as err:
-        if err.winerror == winerror.ERROR_FILE_NOT_FOUND:
+        if err.winerror == winutil.ERROR_FILE_NOT_FOUND:
             return None, None, None
     with k:
         cmd = k.get(sub_key=r'shell\open\command')
@@ -249,7 +249,7 @@ def find_programs(extensions):
         try:
             k = Key(open_at=r'Software\RegisteredApplications', root=base)
         except OSError as err:
-            if err.winerror == winerror.ERROR_FILE_NOT_FOUND:
+            if err.winerror == winutil.ERROR_FILE_NOT_FOUND:
                 continue
             raise
         with k:
@@ -274,7 +274,7 @@ def find_programs(extensions):
         try:
             k = Key(open_at=r'Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.%s\OpenWithProgIDs' % ext, root=HKEY_CURRENT_USER)
         except OSError as err:
-            if err.winerror == winerror.ERROR_FILE_NOT_FOUND:
+            if err.winerror == winutil.ERROR_FILE_NOT_FOUND:
                 continue
         for prog_id in itervalues(k):
             if prog_id and prog_id not in seen_prog_ids: