diff --git a/test/hil/hfp.json b/test/hil/hfp.json
index 735d5a402..17fbb7605 100644
--- a/test/hil/hfp.json
+++ b/test/hil/hfp.json
@@ -36,7 +36,11 @@
             "flasher": {
                 "name": "jlink",
                 "uid": "728973776",
-                "args": "-device LPC43S67_M4"
+                "args": "-device LPC43S67_M4",
+                "pre_flash": {
+                    "device/usbtest": "device/board_test",
+                    "device/cdc_msc_throughput": "device/board_test"
+                }
             }
         }
     ]
diff --git a/test/hil/hil_test.py b/test/hil/hil_test.py
index 0efc6826f..72af6c697 100755
--- a/test/hil/hil_test.py
+++ b/test/hil/hil_test.py
@@ -321,6 +321,7 @@ class FlasherCfg(TypedDict):
     name: str
     uid: str
     args: str
+    pre_flash: NotRequired[dict[str, str]]  # target example -> USB-off separator example
 
 
 class AttachedDevCfg(TypedDict, total=False):
@@ -1830,6 +1831,19 @@ def find_firmware(variant: str, example: str):
     return None
 
 
+def usb_uid_paths(uid: str) -> set[str]:
+    """Return sysfs device paths currently exposing the requested USB serial."""
+    paths = set()
+    for f in glob.glob('/sys/bus/usb/devices/*/serial'):
+        try:
+            with open(f) as serial_file:
+                if serial_file.read().strip().lower() == uid.lower():
+                    paths.add(os.path.dirname(f))
+        except OSError:
+            pass
+    return paths
+
+
 def test_example(board: Board, variant: str, example: str) -> tuple[int, str, str | None]:
     """
     Test example firmware
@@ -1852,7 +1866,16 @@ def test_example(board: Board, variant: str, example: str) -> tuple[int, str, st
         log_line(f'{test_name} Skip (no binary)')
         return 0, 'skip', None
 
+    pre_flash_example = None if skip_flash else board['flasher'].get('pre_flash', {}).get(example)
+    pre_flash_name = find_firmware(variant, pre_flash_example) if pre_flash_example else None
+    if pre_flash_example and pre_flash_name is None:
+        log_line(f'{test_name}  {STATUS_FAILED}: '
+                 f'pre-flash firmware {pre_flash_example} not found')
+        return 1, 'fail', None
+
     if verbose:
+        if pre_flash_name is not None:
+            log_line(f'Pre-flashing {pre_flash_name}.elf')
         log_line(f'Flashing {fw_name}.elf')
 
     # flash firmware (unless --skip-flash), then run the test. Both may fail randomly,
@@ -1867,13 +1890,36 @@ def test_example(board: Board, variant: str, example: str) -> tuple[int, str, st
         attempt_out = io.StringIO()
         with redirect_stdout(attempt_out):
             if not skip_flash:
+                flash_ok = True
+                flash_error = ''
                 with flash_permit(board['uid']):
-                    t_flash = time.monotonic()
-                    ret = globals()[f'flash_{board["flasher"]["name"].lower()}'](board, str(fw_name))
-                    if PROFILE:
-                        log_line(f'[prof] {variant} {example} flash attempt {i + 1}: '
-                                 f'{time.monotonic() - t_flash:.1f}s rc={ret.returncode}')
-                flash_ok = (ret.returncode == 0)
+                    if pre_flash_name is not None:
+                        previous_usb_paths = usb_uid_paths(board['uid'])
+                        ret = globals()[f'flash_{board["flasher"]["name"].lower()}'](
+                            board, str(pre_flash_name))
+                        flash_ok = (ret.returncode == 0)
+                        if not flash_ok:
+                            flash_error = f'Pre-flash {pre_flash_example} failed'
+                        elif previous_usb_paths:
+                            disconnected = wait_until(
+                                lambda: all(not os.path.exists(p) for p in previous_usb_paths), step=0.1)
+                            if not disconnected:
+                                flash_ok = False
+                                flash_error = (f'Pre-flash {pre_flash_example} did not disconnect '
+                                               f'USB device {board["uid"]}')
+                        else:
+                            time.sleep(0.1)
+
+                    if flash_ok:
+                        t_flash = time.monotonic()
+                        ret = globals()[f'flash_{board["flasher"]["name"].lower()}'](
+                            board, str(fw_name))
+                        if PROFILE:
+                            log_line(f'[prof] {variant} {example} flash attempt {i + 1}: '
+                                     f'{time.monotonic() - t_flash:.1f}s rc={ret.returncode}')
+                        flash_ok = (ret.returncode == 0)
+                        if not flash_ok:
+                            flash_error = 'Flash failed'
             if flash_ok:
                 try:
                     tret = globals()[f'test_{example.replace("/", "_")}'](board)
@@ -1911,10 +1957,10 @@ def test_example(board: Board, variant: str, example: str) -> tuple[int, str, st
                         log_line(msg)
                         time.sleep(0.5)
             else:
-                last_err = 'Flash failed'
+                last_err = flash_error
                 last_detail = compact_output(attempt_out.getvalue())
                 if i < max_retry - 1:
-                    msg = f'{test_name}  retry {i+2}/{max_retry}: flash failed'
+                    msg = f'{test_name}  retry {i+2}/{max_retry}: {flash_error}'
                     if last_detail:
                         msg += f'  {last_detail}'
                     log_line(msg)