diff --git a/isr.py b/isr.py
index cc1d903..22fc974 100644
--- a/isr.py
+++ b/isr.py
@@ -714,11 +714,12 @@ class SoundcardRecorder(BaseRecorder):
         if self.current_file is None:
             return
 
+        # Swap the buffer under the lock, write outside it — disk writes (and
+        # FLAC encoding) must not block the capture callback.
         with self.buffer_lock:
-            if self.audio_buffer:
-                for data in self.audio_buffer:
-                    self.current_file.write(data)
-                self.audio_buffer.clear()
+            buffered, self.audio_buffer = self.audio_buffer, []
+        for data in buffered:
+            self.current_file.write(data)
 
     def close_current_file(self):
         """Flush any buffered audio, then close the current recording file."""