diff --git a/board/main.c b/board/main.c
index 9ac7bd90..ffaee3ae 100644
--- a/board/main.c
+++ b/board/main.c
@@ -236,7 +236,8 @@ void usb_cb_ep3_out(void *usbdata, int len, bool hardwired) {
 }
 
 void usb_cb_ep3_out_complete(void) {
-  if (can_tx_check_min_slots_free(MAX_CAN_MSGS_PER_BULK_TRANSFER)) {
+  // TODO: how does a second USB packet sneek in? (why multiply by 2)
+  if (can_tx_check_min_slots_free(MAX_CAN_MSGS_PER_BULK_TRANSFER * 2U)) {
     usb_outep3_resume_if_paused();
   }
 }
diff --git a/python/__init__.py b/python/__init__.py
index 761f4319..579c655a 100644
--- a/python/__init__.py
+++ b/python/__init__.py
@@ -533,7 +533,13 @@ class Panda(object):
           for s in snds:
             self._handle.bulkWrite(3, s)
         else:
-          self._handle.bulkWrite(3, b''.join(snds), timeout=timeout)
+          dat = b''.join(snds)
+          while True:
+            bs = self._handle.bulkWrite(3, dat, timeout=timeout)
+            dat = dat[bs:]
+            if len(dat) == 0:
+              break
+            print("CAN: PARTIAL SEND MANY, RETRYING")
         break
       except (usb1.USBErrorIO, usb1.USBErrorOverflow):
         print("CAN: BAD SEND MANY, RETRYING")
diff --git a/tests/automated/4_can_loopback.py b/tests/automated/4_can_loopback.py
index b069d6f0..923d3635 100644
--- a/tests/automated/4_can_loopback.py
+++ b/tests/automated/4_can_loopback.py
@@ -214,8 +214,12 @@ def test_bulk_write(p):
 
   def flood_tx(panda):
     print('Sending!')
-    msg = b"\xaa" * 4
-    packet = [[0xaa, None, msg, 0], [0xaa, None, msg, 1], [0xaa, None, msg, 2]] * NUM_MESSAGES_PER_BUS
+    msg = b"\xaa" * 8
+    packet = []
+    # start with many messages on a single bus (higher contention for single TX ring buffer)
+    packet += [[0xaa, None, msg, 0]] * NUM_MESSAGES_PER_BUS
+    # end with many messages on multiple buses
+    packet += [[0xaa, None, msg, 0], [0xaa, None, msg, 1], [0xaa, None, msg, 2]] * NUM_MESSAGES_PER_BUS
 
     # Disable timeout
     panda.can_send_many(packet, timeout=0)
@@ -241,7 +245,7 @@ def test_bulk_write(p):
   print(f"Received {len(rx)} messages")
 
   # All messages should have been received
-  if len(rx) != 3 * NUM_MESSAGES_PER_BUS:
+  if len(rx) != 4 * NUM_MESSAGES_PER_BUS:
     Exception("Did not receive all messages!")
 
   # Set back to silent mode