diff --git a/examples/sd_drive.py b/examples/sd_drive.py
index 5f854d1..3206a38 100644
--- a/examples/sd_drive.py
+++ b/examples/sd_drive.py
@@ -69,6 +69,28 @@ def open_card():
         ) from exc
 
 
+def card_geometry(card):
+    """Return ``(blocks, block_size)``, or raise if the card never answered.
+
+    ``ioctl(4)`` and ``ioctl(5)`` are the block count and block size, and a
+    card that is absent, unpowered or still timing out answers them with
+    ``-1`` rather than raising. Serving that to a host is serving a drive of
+    no blocks: the example used to print a negative size and then do nothing
+    at all, which reads as the example being broken rather than the card
+    (usbif#4). So refuse it here, while the reason is still known.
+    """
+    blocks = card.ioctl(4, 0)
+    block_size = card.ioctl(5, 0)
+    if not isinstance(blocks, int) or not isinstance(block_size, int) \
+            or blocks <= 0 or block_size <= 0:
+        raise RuntimeError(
+            "the SD card did not answer (ioctl reported {} blocks of {} "
+            "bytes): check that a card is seated in the slot, then reset "
+            "the board, since a card left half-initialised by an earlier "
+            "attempt does not recover on its own".format(blocks, block_size))
+    return blocks, block_size
+
+
 def main():
     dev = usbif.auto.device()
 
@@ -78,8 +100,7 @@ def main():
     dev.msc_detach()
 
     card = open_card()
-    blocks = card.ioctl(4, 0)
-    block_size = card.ioctl(5, 0)
+    blocks, block_size = card_geometry(card)
     print("card: {} blocks x {} bytes = {} MB".format(
         blocks, block_size, blocks * block_size // (1024 * 1024)))
 
diff --git a/tests/test_sd_drive_example.py b/tests/test_sd_drive_example.py
new file mode 100644
index 0000000..1ac3166
--- /dev/null
+++ b/tests/test_sd_drive_example.py
@@ -0,0 +1,99 @@
+# SPDX-FileCopyrightText: 2026 Brad Barnett
+#
+# SPDX-License-Identifier: MIT
+"""examples/sd_drive.py must refuse a card that never answered (usbif#4).
+
+A card that is absent or timing out answers ``ioctl(4)``/``ioctl(5)`` with -1
+instead of raising. The example used to hand that to the host as a drive of
+no blocks and then sit in its service loop, silent. These tests run the
+example's ``main()`` against a fake card and a fake device, so the refusal is
+checked on the path a user actually takes, with a healthy card as the control
+that must still attach.
+"""
+
+import importlib.util
+import pathlib
+import types
+import unittest
+
+import _env  # noqa: F401
+
+import usbif.auto
+
+ROOT = pathlib.Path(__file__).resolve().parents[1]
+
+
+def _load_example():
+    spec = importlib.util.spec_from_file_location(
+        "sd_drive_example", ROOT / "examples" / "sd_drive.py")
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    # CPython's time has no sleep_ms; the service loop only needs it to exist.
+    module.time = types.SimpleNamespace(sleep_ms=lambda ms: None)
+    return module
+
+
+class _Card:
+    def __init__(self, blocks, block_size):
+        self._answers = {4: blocks, 5: block_size}
+
+    def ioctl(self, op, arg):
+        return self._answers.get(op)
+
+
+class _Device:
+    """Records what the example asked of the USB device."""
+
+    def __init__(self):
+        self.attached = None
+
+    def msc_detach(self):
+        pass
+
+    def msc_attach_blockdev(self, card, writable):
+        self.attached = card
+
+    def functions(self, *names):
+        pass
+
+    def msc_status(self):
+        # Attached, and already ejected, so the service loop ends at once.
+        return (self.attached is not None, 0, True)
+
+    def msc_bd_stats(self):
+        return (0, 0)
+
+
+class SdDriveRefusesDeadCard(unittest.TestCase):
+    def setUp(self):
+        self.example = _load_example()
+        self.dev = _Device()
+        self._saved_device = usbif.auto.device
+        usbif.auto.device = lambda: self.dev
+
+    def tearDown(self):
+        usbif.auto.device = self._saved_device
+
+    def _run_with(self, card):
+        self.example.open_card = lambda: card
+        self.example.main()
+
+    def test_card_that_never_answered_is_refused_before_attach(self):
+        for blocks, size in ((-1, -1), (0, 512), (62685184, -1), (None, 512)):
+            with self.subTest(blocks=blocks, size=size):
+                self.dev.attached = None
+                with self.assertRaises(RuntimeError) as ctx:
+                    self._run_with(_Card(blocks, size))
+                self.assertIn("did not answer", str(ctx.exception))
+                self.assertIsNone(
+                    self.dev.attached,
+                    "a card with no geometry was served to the host")
+
+    def test_healthy_card_still_attaches(self):
+        card = _Card(62685184, 512)
+        self._run_with(card)
+        self.assertIs(self.dev.attached, card)
+
+
+if __name__ == "__main__":
+    unittest.main()