diff --git a/Downstream/Inc/downstream_statemachine.h b/Downstream/Inc/downstream_statemachine.h
index cafc548..ed644d0 100644
--- a/Downstream/Inc/downstream_statemachine.h
+++ b/Downstream/Inc/downstream_statemachine.h
@@ -32,6 +32,13 @@ typedef enum
 } while (0);
 
 
+#define DOWNSTREAM_STATEMACHINE_FREAKOUT_NORETURN		\
+	do {												\
+		LED_Fault_SetBlinkRate(LED_FAST_BLINK_RATE);	\
+		DownstreamState = STATE_ERROR;					\
+} while (0);
+
+
 
 
 void Downstream_InitStateMachine(void);
diff --git a/Downstream/Middlewares/ST/STM32_USB_Host_Library/Class/MSC/Src/usbh_msc.c b/Downstream/Middlewares/ST/STM32_USB_Host_Library/Class/MSC/Src/usbh_msc.c
index 522bef6..d91988b 100644
--- a/Downstream/Middlewares/ST/STM32_USB_Host_Library/Class/MSC/Src/usbh_msc.c
+++ b/Downstream/Middlewares/ST/STM32_USB_Host_Library/Class/MSC/Src/usbh_msc.c
@@ -42,6 +42,7 @@
 #include "usbh_msc.h"
 #include "usbh_msc_bot.h"    
 #include "usbh_msc_scsi.h"
+#include "interrupts.h"
 
 
 /** @addtogroup USBH_LIB
@@ -498,6 +499,10 @@ static USBH_StatusTypeDef USBH_MSC_Process(USBH_HandleTypeDef *phost)
 
   case MSC_READ:
   case MSC_WRITE:
+	//USBH_MSC_RdWrProcess interacts heavily with downstream SPI code.
+	//So to protect against preemption we elevate our priority here.
+	__set_BASEPRI(INT_PRIORITY_OTG_FS);
+
 	error = USBH_MSC_RdWrProcess(phost, MSC_Handle->rw_lun);
 	if(((int32_t)(phost->Timer - MSC_Handle->timeout) > 0) || (phost->device.is_connected == 0))
 	{
@@ -513,6 +518,8 @@ static USBH_StatusTypeDef USBH_MSC_Process(USBH_HandleTypeDef *phost)
 			MSC_Handle->RdWrCompleteCallback = NULL;
 		}
 	}
+
+	__set_BASEPRI(0);
 	break;
 
   default:
diff --git a/Downstream/Middlewares/ST/STM32_USB_Host_Library/Class/MSC/Src/usbh_msc_bot.c b/Downstream/Middlewares/ST/STM32_USB_Host_Library/Class/MSC/Src/usbh_msc_bot.c
index 2b1615c..5284cae 100644
--- a/Downstream/Middlewares/ST/STM32_USB_Host_Library/Class/MSC/Src/usbh_msc_bot.c
+++ b/Downstream/Middlewares/ST/STM32_USB_Host_Library/Class/MSC/Src/usbh_msc_bot.c
@@ -388,6 +388,12 @@ void USBH_MSC_BOT_Read_FreePacketCallback(DownstreamPacketTypeDef* freePacket)
 {
 	MSC_HandleTypeDef *MSC_Handle =  (MSC_HandleTypeDef *) Callback_MSC_phost->pActiveClass->pData;
 
+	if (MSC_Handle->hbot.state != BOT_DATA_IN_WAIT)
+	{
+		Downstream_PacketProcessor_FreakOut();
+		return;
+	}
+
 	MSC_Handle->hbot.bot_packet = freePacket;
 	MSC_Handle->hbot.pbuf = freePacket->Data;
 	MSC_Handle->hbot.bot_packet_bytes_remaining = BOT_PAGE_LENGTH;
@@ -421,6 +427,12 @@ void USBH_MSC_BOT_Write_ReceivePacketCallback(DownstreamPacketTypeDef* receivedP
 {
 	MSC_HandleTypeDef *MSC_Handle =  (MSC_HandleTypeDef *) Callback_MSC_phost->pActiveClass->pData;
 
+	if (MSC_Handle->hbot.state != BOT_DATA_OUT_WAIT)
+	{
+		Downstream_PacketProcessor_FreakOut();
+		return;
+	}
+
 	MSC_Handle->hbot.bot_packet = receivedPacket;
 	MSC_Handle->hbot.pbuf = receivedPacket->Data;
 	MSC_Handle->hbot.bot_packet_bytes_remaining = dataLength;
diff --git a/Downstream/Src/downstream_statemachine.c b/Downstream/Src/downstream_statemachine.c
index c78b8eb..157ead3 100644
--- a/Downstream/Src/downstream_statemachine.c
+++ b/Downstream/Src/downstream_statemachine.c
@@ -13,6 +13,7 @@
 #include "usbh_core.h"
 #include "usbh_msc.h"
 #include "led.h"
+#include "interrupts.h"
 
 
 
@@ -153,9 +154,9 @@ void Downstream_PacketProcessor_ClassReply(DownstreamPacketTypeDef* replyPacket)
 }
 
 
-//This callback receives various event ids from the host stack, either
-//at INT_PRIORITY_OTG_FS or from main(). We should therefore be prepared
-//for pre-emption by USB or SPI/DMA interrupts.
+//This callback receives various event ids from the host stack,
+//either at INT_PRIORITY_OTG_FS or from main().
+//We should therefore elevate our execution priority to INT_PRIORITY_OTG_FS where necessary.
 void Downstream_HostUserCallback(USBH_HandleTypeDef *phost, uint8_t id)
 {
 	InterfaceCommandClassTypeDef newActiveClass = COMMAND_CLASS_INTERFACE;
@@ -172,7 +173,19 @@ void Downstream_HostUserCallback(USBH_HandleTypeDef *phost, uint8_t id)
 		return;
 	}
 
-	//Called from main(). Beware pre-emption!
+	//Elevate our priority level so we aren't interrupted
+	__set_BASEPRI(INT_PRIORITY_OTG_FS);
+
+
+	//Called from main()
+	if (id == HOST_USER_UNRECOVERED_ERROR)
+	{
+		DOWNSTREAM_STATEMACHINE_FREAKOUT_NORETURN;
+		__set_BASEPRI(0);
+		return;
+	}
+
+	//Called from main()
 	if (id == HOST_USER_CLASS_ACTIVE)
 	{
 		switch (phost->pActiveClass->ClassCode)
@@ -186,11 +199,12 @@ void Downstream_HostUserCallback(USBH_HandleTypeDef *phost, uint8_t id)
 
 		//Add other classes here...
 
-		//Any unsupported device will cause a slow fault flash.
+		//Unsupported device classes will cause a slow fault flash.
 		//This is distinct from the fast freakout flash caused by internal errors or attacks.
 		default:
 			LED_Fault_SetBlinkRate(LED_SLOW_BLINK_RATE);
 			DownstreamState = STATE_ERROR;
+			__set_BASEPRI(0);
 			return;
 		}
 
@@ -198,7 +212,9 @@ void Downstream_HostUserCallback(USBH_HandleTypeDef *phost, uint8_t id)
 		//If the new device has failed its 'approval' checks, we are sufficiently freaked out.
 		if (newActiveClass == COMMAND_CLASS_INTERFACE)
 		{
-			DOWNSTREAM_STATEMACHINE_FREAKOUT;
+			DOWNSTREAM_STATEMACHINE_FREAKOUT_NORETURN;
+			__set_BASEPRI(0);
+			return;
 		}
 
 		//If we already configured a device class, we cannot change to a different one without rebooting.
@@ -206,23 +222,29 @@ void Downstream_HostUserCallback(USBH_HandleTypeDef *phost, uint8_t id)
 		if ((ConfiguredDeviceClass != COMMAND_CLASS_INTERFACE) &&
 			(ConfiguredDeviceClass != newActiveClass))
 		{
-			DOWNSTREAM_STATEMACHINE_FREAKOUT;
+			DOWNSTREAM_STATEMACHINE_FREAKOUT_NORETURN;
+			__set_BASEPRI(0);
+			return;
 		}
 		ConfiguredDeviceClass = newActiveClass;
 
 		if (DownstreamState == STATE_WAIT_DEVICE_READY)
 		{
 			Downstream_GetFreePacket(Downstream_PacketProcessor_Interface_ReplyNotifyDevice);
+			__set_BASEPRI(0);
 			return;
 		}
 
 		if (DownstreamState == STATE_DEVICE_NOT_READY)
 		{
 			DownstreamState = STATE_DEVICE_READY;
+			__set_BASEPRI(0);
 			return;
 		}
 
-		DOWNSTREAM_STATEMACHINE_FREAKOUT;
+		DOWNSTREAM_STATEMACHINE_FREAKOUT_NORETURN;
+		__set_BASEPRI(0);
+		return;
 	}
 }