diff --git a/src/calibre/devices/usbms/device.py b/src/calibre/devices/usbms/device.py
index 5c7bf5c25b..1708956546 100644
--- a/src/calibre/devices/usbms/device.py
+++ b/src/calibre/devices/usbms/device.py
@@ -248,14 +248,15 @@ class Device(DeviceConfig, DevicePlugin):
         filtered = set()
         for dl in dlmap['drive_letters']:
             pnp_id = dlmap['pnp_id_map'][dl].upper()
-            if self.windows_filter_pnp_id(pnp_id):
+            if dl in dlmap['readonly_drives']:
+                filtered.add(dl)
+                if debug:
+                    prints('Ignoring the drive %s as it is readonly' % dl)
+            elif self.windows_filter_pnp_id(pnp_id):
                 filtered.add(dl)
                 if debug:
                     prints('Ignoring the drive %s because of a PNP filter on %s' % (dl, pnp_id))
-        dlmap['drive_letters'] = [dl for dl in dlmap['drive_letters'] if dl not in filtered]
-        filtered = set()
-        for dl in dlmap['drive_letters']:
-            if not drive_is_ok(dl, debug=debug):
+            elif not drive_is_ok(dl, debug=debug):
                 filtered.add(dl)
                 if debug:
                     prints('Ignoring the drive %s because failed to get free space for it' % dl)