Index: mtools-4.0.33-1+really4.0.32/fat.c
===================================================================
--- mtools-4.0.33-1+really4.0.32.orig/fat.c
+++ mtools-4.0.33-1+really4.0.32/fat.c
@@ -602,8 +602,13 @@ static void set_fat32(Fs_t *This)
 	}
 }
 
-void set_fat(Fs_t *This) {
-	if(This->num_clus < FAT12)
+void set_fat(Fs_t *This, bool haveBigFatLen) {
+	if(haveBigFatLen) {
+		/* This is how Windows 10 behaves, despite
+		   fatgen103's stern assertion otherwise */
+		set_fat32(This);
+	}
+	else if(This->num_clus < FAT12)
 		set_fat12(This);
 	else if(This->num_clus < FAT16)
 		set_fat16(This);
@@ -767,7 +772,7 @@ int fat_read(Fs_t *This, union bootsecto
 	This->lastFatSectorNr = 0;
 	This->lastFatSectorData = 0;
 
-	if(This->num_clus < FAT16)
+	if(This->fat_bits <= 16)
 		return old_fat_read(This, boot, nodups);
 	else
 		return fat_32_read(This, boot);
Index: mtools-4.0.33-1+really4.0.32/fsP.h
===================================================================
--- mtools-4.0.33-1+really4.0.32.orig/fsP.h
+++ mtools-4.0.33-1+really4.0.32/fsP.h
@@ -88,7 +88,7 @@ typedef struct Fs_t {
 
 int fs_free(Stream_t *Stream);
 
-void set_fat(Fs_t *This);
+void set_fat(Fs_t *This, bool haveBigFatLen);
 unsigned int get_next_free_cluster(Fs_t *Fs, unsigned int last);
 unsigned int fatDecode(Fs_t *This, unsigned int pos);
 void fatAppend(Fs_t *This, unsigned int pos, unsigned int newpos);
Index: mtools-4.0.33-1+really4.0.32/init.c
===================================================================
--- mtools-4.0.33-1+really4.0.32.orig/init.c
+++ mtools-4.0.33-1+really4.0.32/init.c
@@ -397,6 +397,7 @@ uint32_t parseFsParams(	Fs_t *This,
 			unsigned int cylinder_size)
 {
 	uint32_t tot_sectors;
+	int haveBigFatLen = 0;
 
 	if ((media & ~7) == 0xf8){
 		/* This bit of code is only entered if there is no BPB, or
@@ -456,6 +457,7 @@ uint32_t parseFsParams(	Fs_t *This,
 		} else {
 			labelBlock = &boot->boot.ext.fat32.labelBlock;
 			This->fat_len = DWORD(ext.fat32.bigFat);
+			haveBigFatLen = 1;
 			This->backupBoot = WORD(ext.fat32.backupBoot);
 		}
 
@@ -468,7 +470,7 @@ uint32_t parseFsParams(	Fs_t *This,
 	if(calc_num_clus(This, tot_sectors) < 0)
 		/* Too few sectors */
 		return 0;
-	set_fat(This);
+	set_fat(This, haveBigFatLen);
 
 	return tot_sectors;
 }
Index: mtools-4.0.33-1+really4.0.32/mformat.c
===================================================================
--- mtools-4.0.33-1+really4.0.32.orig/mformat.c
+++ mtools-4.0.33-1+really4.0.32/mformat.c
@@ -367,7 +367,7 @@ static void check_fs_params_and_set_fat(
 	assert(clusters_fit_into_fat(Fs));
 #endif
 	provisional_fat_bits = Fs->fat_bits;
-	set_fat(Fs);
+	set_fat(Fs, provisional_fat_bits == 32);
 #ifdef HAVE_ASSERT_H
 	assert(provisional_fat_bits == Fs->fat_bits);
 #endif