diff --git a/ports/atmel-samd/asf4_conf/samd51/hpl_nvmctrl_config.h b/ports/atmel-samd/asf4_conf/samd51/hpl_nvmctrl_config.h
index daaba85d4018c..53fcb593abbc4 100644
--- a/ports/atmel-samd/asf4_conf/samd51/hpl_nvmctrl_config.h
+++ b/ports/atmel-samd/asf4_conf/samd51/hpl_nvmctrl_config.h
@@ -19,14 +19,14 @@
 // <i> Indicate whether AHB0 cache is disable or not
 // <id> nvm_arch_cache0
 #ifndef CONF_NVM_CACHE0
-#define CONF_NVM_CACHE0 0
+#define CONF_NVM_CACHE0 1
 #endif
 
 // <q> AHB1 Cache Disable
 // <i> Indicate whether AHB1 cache is disable or not
 // <id> nvm_arch_cache1
 #ifndef CONF_NVM_CACHE1
-#define CONF_NVM_CACHE1 0
+#define CONF_NVM_CACHE1 1
 #endif
 
 // </h>
diff --git a/ports/atmel-samd/usb_mass_storage.c b/ports/atmel-samd/usb_mass_storage.c
index d9d9d9f39384e..85c662e774c93 100644
--- a/ports/atmel-samd/usb_mass_storage.c
+++ b/ports/atmel-samd/usb_mass_storage.c
@@ -63,10 +63,10 @@ static fs_user_mount_t* get_vfs(int lun) {
 /* Inquiry Information */
 // This is designed to handle the common case where we have an internal file
 // system and an optional SD card.
-static uint8_t inquiry_info[2][36];
+COMPILER_ALIGNED(4) static uint8_t inquiry_info[2][36];
 
 /* Capacities of Disk */
-static uint8_t format_capa[2][8];
+COMPILER_ALIGNED(4) static uint8_t format_capa[2][8];
 
 /**
  * \brief Eject Disk
@@ -256,18 +256,17 @@ int32_t usb_msc_xfer_done(uint8_t lun) {
         return ERR_DENIED;
     }
 
+    CRITICAL_SECTION_ENTER();
     if (active_read) {
         active_addr += 1;
         active_nblocks--;
-        if (active_nblocks == 0) {
-            active_read = false;
-        }
     }
 
     if (active_write) {
         sector_loaded = true;
     }
     usb_busy = false;
+    CRITICAL_SECTION_LEAVE();
 
     return ERR_NONE;
 }
@@ -275,11 +274,16 @@ int32_t usb_msc_xfer_done(uint8_t lun) {
 // The start_read callback begins a read transaction which we accept but delay our response until the "main thread" calls usb_msc_background. Once it does, we read immediately from the drive into our cache and trigger the USB DMA to output the sector. Once the sector is transmitted, xfer_done will be called.
 void usb_msc_background(void) {
     if (active_read && !usb_busy) {
+        if (active_nblocks == 0) {
+            active_read = false;
+            return;
+        }
         fs_user_mount_t * vfs = get_vfs(active_lun);
         disk_read(vfs, sector_buffer, active_addr, 1);
-        // TODO(tannewt): Check the read result.
-        mscdf_xfer_blocks(true, sector_buffer, 1);
-        usb_busy = true;
+        CRITICAL_SECTION_ENTER();
+        int32_t result = mscdf_xfer_blocks(true, sector_buffer, 1);
+        usb_busy = result == ERR_NONE;
+        CRITICAL_SECTION_LEAVE();
     }
     if (active_write && !usb_busy) {
         if (sector_loaded) {
@@ -306,8 +310,14 @@ void usb_msc_background(void) {
         }
         // Load more blocks from USB if they are needed.
         if (active_nblocks > 0) {
+            // Turn off interrupts because with them on,
+            // usb_msc_xfer_done could be called before we update
+            // usb_busy. If that happened, we'd overwrite the fact that
+            // the transfer actually already finished.
+            CRITICAL_SECTION_ENTER();
             int32_t result = mscdf_xfer_blocks(false, sector_buffer, 1);
-            usb_busy = result != ERR_NONE;
+            usb_busy = result == ERR_NONE;
+            CRITICAL_SECTION_LEAVE();
         } else {
             mscdf_xfer_blocks(false, NULL, 0);
             active_write = false;