diff --git a/block/vvfat.c b/block/vvfat.c
index 3c0ab6c228..cb124d4650 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -39,6 +39,7 @@
 #include "qemu/ctype.h"
 #include "qemu/cutils.h"
 #include "qemu/error-report.h"
+#include "qemu/units.h"
 
 #ifndef S_IWGRP
 #define S_IWGRP 0
@@ -336,6 +337,9 @@ typedef struct BDRVVVFATState {
     int downcase_short_names;
 
     Error *migration_blocker;
+#ifdef EMSCRIPTEN
+    GHashTable *write_overlay;
+#endif
 } BDRVVVFATState;
 
 /* take the sector position spos and convert it to Cylinder/Head/Sector position
@@ -1246,10 +1250,13 @@ static int vvfat_open(BlockDriverState *bs, QDict *options, int flags,
     s->bs = bs;
 
 #ifdef EMSCRIPTEN
-    if (!floppy && s->fat_type == 32) {
+    if (!floppy && s->fat_type == 16) {
+        s->sectors_per_cluster = 0x10;
+        total_sectors = 32ULL * MiB / BDRV_SECTOR_SIZE;
+    } else if (!floppy && s->fat_type == 32) {
         /* Browser SD Geometry - A larger cluster keeps the synthesized 32 GiB FAT near 4 MiB. */
         s->sectors_per_cluster = 0x40;
-        total_sectors = 32ULL * 1024 * 1024 * 1024 / BDRV_SECTOR_SIZE;
+        total_sectors = 32ULL * GiB / BDRV_SECTOR_SIZE;
     } else
 #endif
     {
@@ -1516,6 +1523,18 @@ vvfat_read(BlockDriverState *bs, int64_t sector_num, uint8_t *buf, int nb_sector
     for(i=0;i<nb_sectors;i++,sector_num++) {
         if (sector_num >= bs->total_sectors)
            return -1;
+#ifdef EMSCRIPTEN
+        if (s->write_overlay) {
+            int64_t key = sector_num;
+            const uint8_t *overlay = g_hash_table_lookup(s->write_overlay,
+                                                         &key);
+            if (overlay) {
+                memcpy(buf + i * BDRV_SECTOR_SIZE, overlay,
+                       BDRV_SECTOR_SIZE);
+                continue;
+            }
+        }
+#endif
         if (s->qcow) {
             int64_t n;
             int ret;
@@ -3027,6 +3046,19 @@ vvfat_write(BlockDriverState *bs, int64_t sector_num,
 
 DLOG(checkpoint());
 
+#ifdef EMSCRIPTEN
+    if (s->write_overlay) {
+        for (i = 0; i < nb_sectors; i++) {
+            int64_t *key = g_new(int64_t, 1);
+            uint8_t *sector = g_malloc(BDRV_SECTOR_SIZE);
+            *key = sector_num + i;
+            memcpy(sector, buf + i * BDRV_SECTOR_SIZE, BDRV_SECTOR_SIZE);
+            g_hash_table_replace(s->write_overlay, key, sector);
+        }
+        return 0;
+    }
+#endif
+
     /* Check if we're operating in read-only mode */
     if (s->qcow == NULL) {
         return -EACCES;
@@ -3129,7 +3161,8 @@ DLOG(fprintf(stderr, "Write to qcow backend: %d + %d\n", (int)sector_num, nb_sec
     ret = bdrv_co_pwrite(s->qcow, sector_num * BDRV_SECTOR_SIZE,
                          nb_sectors * BDRV_SECTOR_SIZE, buf, 0);
     if (ret < 0) {
-        fprintf(stderr, "Error writing to qcow backend\n");
+        fprintf(stderr, "Error writing to qcow backend: %s (%d)\n",
+                strerror(-ret), ret);
         return ret;
     }
 
@@ -3205,6 +3238,12 @@ static int enable_write_target(BlockDriverState *bs, Error **errp)
     int size = sector2cluster(s, s->sector_count);
     QDict *options;
 
+#ifdef EMSCRIPTEN
+    s->write_overlay = g_hash_table_new_full(g_int64_hash, g_int64_equal,
+                                             g_free, g_free);
+    return 0;
+#endif
+
     s->used_clusters = g_malloc0(size);
 
     array_init(&(s->commits), sizeof(commit_t));
@@ -3280,6 +3319,9 @@ static void vvfat_close(BlockDriverState *bs)
     if (s->qcow) {
         migrate_del_blocker(&s->migration_blocker);
     }
+#ifdef EMSCRIPTEN
+    g_clear_pointer(&s->write_overlay, g_hash_table_destroy);
+#endif
 }
 
 static const char *const vvfat_strong_runtime_opts[] = {