diff --git a/examples/device/nrf52840/src/msc_device_app.c b/examples/device/nrf52840/src/msc_device_app.c
index aa6b2fb43..96dea077e 100644
--- a/examples/device/nrf52840/src/msc_device_app.c
+++ b/examples/device/nrf52840/src/msc_device_app.c
@@ -57,6 +57,9 @@ void msc_app_umount(uint8_t rhport)
 
 }
 
+// Callback invoked when received an SCSI command not in built-in list below
+// - READ_CAPACITY10, READ_FORMAT_CAPACITY, INQUIRY, MODE_SENSE6, REQUEST_SENSE
+// - READ10 and WRITE10 has their own callbacks
 int32_t tud_msc_scsi_cb (uint8_t lun, uint8_t const scsi_cmd[16], void* buffer, uint16_t bufsize)
 {
   // read10 & write10 has their own callback and MUST not be handled here
@@ -80,12 +83,12 @@ int32_t tud_msc_scsi_cb (uint8_t lun, uint8_t const scsi_cmd[16], void* buffer,
     break;
 
     case SCSI_CMD_START_STOP_UNIT:
-      // Host try to eject/safe remove/powerof us. We could safely disconnect with disk storage, or go into lower power
+      // Host try to eject/safe remove/poweroff us. We could safely disconnect with disk storage, or go into lower power
       // scsi_start_stop_unit_t const * cmd_start_stop = (scsi_start_stop_unit_t const *) scsi_cmd
       len = 0;
     break;
 
-    // negative is error -> Data stage is STALL, status = failed
+    // negative means error -> tusb could stall and/or response with failed status
     default: return -1;
   }
 
diff --git a/examples/device/nrf52840/src/msc_device_ramdisk.c b/examples/device/nrf52840/src/msc_device_ramdisk.c
index 3d2fca1d6..91ef442db 100644
--- a/examples/device/nrf52840/src/msc_device_ramdisk.c
+++ b/examples/device/nrf52840/src/msc_device_ramdisk.c
@@ -88,9 +88,9 @@ uint8_t msc_device_ramdisk[DISK_BLOCK_NUM][DISK_BLOCK_SIZE] =
   [3] = README_CONTENTS
 };
 
-//--------------------------------------------------------------------+
-// IMPLEMENTATION
-//--------------------------------------------------------------------+
+
+// Callback invoked when received READ10 command.
+// Copy disk's data to buffer (up to bufsize) and return number of copied bytes.
 int32_t tud_msc_read10_cb(uint8_t lun, uint32_t lba, uint32_t offset, void* buffer, uint32_t bufsize)
 {
   uint8_t* addr = msc_device_ramdisk[lba] + offset;
@@ -99,6 +99,8 @@ int32_t tud_msc_read10_cb(uint8_t lun, uint32_t lba, uint32_t offset, void* buff
   return bufsize;
 }
 
+// Callback invoked when received WRITE10 command.
+// Process data in buffer to disk's storage and return number of written bytes
 int32_t tud_msc_write10_cb(uint8_t lun, uint32_t lba, uint32_t offset, void* buffer, uint32_t bufsize)
 {
   uint8_t* addr = msc_device_ramdisk[lba] + offset;