diff --git a/src/usb/msc_uf2.c b/src/usb/msc_uf2.c
index 4682154..977ca6c 100644
--- a/src/usb/msc_uf2.c
+++ b/src/usb/msc_uf2.c
@@ -63,7 +63,7 @@ int write_block(uint32_t block_no, uint8_t *data, bool quiet, WriteState *state)
 int32_t tud_msc_scsi_cb (uint8_t lun, uint8_t const scsi_cmd[16], void* buffer, uint16_t bufsize)
 {
   void const* response = NULL;
-  uint16_t resplen = 0;
+  int32_t resplen = 0;
   memset(buffer, 0, bufsize);
 
   switch ( scsi_cmd[0] )
@@ -99,7 +99,7 @@ int32_t tud_msc_scsi_cb (uint8_t lun, uint8_t const scsi_cmd[16], void* buffer,
   }
 
   // return len must not larger than bufsize
-  if ( resplen > bufsize ) resplen = bufsize;
+  if ( resplen > (int32_t)bufsize ) resplen = bufsize;
 
   // copy response to stack's buffer if any
   if ( response && resplen )
diff --git a/src/usb/uf2/ghostfat.c b/src/usb/uf2/ghostfat.c
index 1f41c10..65ec674 100644
--- a/src/usb/uf2/ghostfat.c
+++ b/src/usb/uf2/ghostfat.c
@@ -110,6 +110,7 @@ static FAT_BootBlock const BootBlock = {
     .SectorsPerFAT        = SECTORS_PER_FAT,
     .SectorsPerTrack      = 1,
     .Heads                = 1,
+	.PhysicalDriveNum     = 0x80, // to match MediaDescriptor of 0xF8
     .ExtendedBootSig      = 0x29,
     .VolumeSerialNumber   = 0x00420042,
     .VolumeLabel          = VOLUME_LABEL,
@@ -123,35 +124,37 @@ static FAT_BootBlock const BootBlock = {
 static uint32_t current_flash_size(void)
 {
   static uint32_t flash_sz = 0;
+  uint32_t result = flash_sz; // presumes atomic 32-bit read/write and static result
 
   // only need to compute once
-  if ( flash_sz == 0 )
+  if ( result == 0 )
   {
     // return 1 block of 256 bytes
     if ( !bootloader_app_is_valid(DFU_BANK_0_REGION_START) )
     {
-      flash_sz = 256;
+      result = 256;
     }else
     {
       bootloader_settings_t const * boot_setting;
       bootloader_util_settings_get(&boot_setting);
 
-      flash_sz = boot_setting->bank_0_size;
+      result = boot_setting->bank_0_size;
 
       // Copy size must be multiple of 256 bytes
       // else we will got an issue copying current.uf2
-      if (flash_sz & 0xff)
+      if (result & 0xff)
       {
-        flash_sz = (flash_sz & ~0xff) + 256;
+        result = (result & ~0xff) + 256;
       }
 
       // if bank0 size is not valid, happens when flashed with jlink
       // use maximum application size
-      if ( (flash_sz == 0) || (flash_sz == 0xFFFFFFFFUL) )
+      if ( (result == 0) || (result == 0xFFFFFFFFUL) )
       {
-        flash_sz = FLASH_SIZE;
+        result = FLASH_SIZE;
       }
     }
+    flash_sz = result; // presumes atomic 32-bit read/write and static result
   }
 
   return flash_sz;
@@ -176,30 +179,30 @@ void read_block(uint32_t block_no, uint8_t *data) {
     memset(data, 0, 512);
     uint32_t sectionIdx = block_no;
 
-    if (block_no == 0) {
+    if (block_no == 0) { // Requested boot block
         memcpy(data, &BootBlock, sizeof(BootBlock));
         data[510] = 0x55;
         data[511] = 0xaa;
         // logval("data[0]", data[0]);
-    } else if (block_no < START_ROOTDIR) {
+    } else if (block_no < START_ROOTDIR) {  // Requested FAT table sector
         sectionIdx -= START_FAT0;
         // logval("sidx", sectionIdx);
         if (sectionIdx >= SECTORS_PER_FAT)
-            sectionIdx -= SECTORS_PER_FAT;
+            sectionIdx -= SECTORS_PER_FAT; // second FAT is same as the first...
         if (sectionIdx == 0) {
-            data[0] = 0xf0;
+            data[0] = 0xf8; // first FAT entry must match BPB MediaDescriptor
             for (int i = 1; i < NUM_INFO * 2 + 4; ++i) {
                 data[i] = 0xff;
             }
         }
-        for (int i = 0; i < 256; ++i) {
+        for (int i = 0; i < 256; ++i) { // Generate the FAT chain for the firmware "file"
             uint32_t v = sectionIdx * 256 + i;
             if (UF2_FIRST_SECTOR <= v && v <= UF2_LAST_SECTOR)
                 ((uint16_t *)(void *)data)[i] = v == UF2_LAST_SECTOR ? 0xffff : v + 1;
         }
-    } else if (block_no < START_CLUSTERS) {
+    } else if (block_no < START_CLUSTERS) { // Requested root directory sector
         sectionIdx -= START_ROOTDIR;
-        if (sectionIdx == 0) {
+        if (sectionIdx == 0) { // only one sector of directory entries generated
             DirEntry *d = (void *)data;
             padded_memcpy(d->name, (char const *) BootBlock.VolumeLabel, 11);
             d->attrs = 0x28;
@@ -209,9 +212,9 @@ void read_block(uint32_t block_no, uint8_t *data) {
                 d->size = inf->content ? strlen(inf->content) : UF2_SIZE;
                 d->startCluster = i + 2;
                 padded_memcpy(d->name, inf->name, 11);
-            }
-        }
-    } else {
+			}
+		}
+    } else { // else Generate the UF2 file data on-the-fly
         sectionIdx -= START_CLUSTERS;
         if (sectionIdx < NUM_INFO - 1) {
             memcpy(data, info[sectionIdx].content, strlen(info[sectionIdx].content));