Index: uspace/app/mkexfat/mkexfat.c
===================================================================
--- uspace/app/mkexfat/mkexfat.c	(revision 0f225288ef9788287ec0fcc0e8997e8a8585189c)
+++ uspace/app/mkexfat/mkexfat.c	(revision ebea7acff6e58e78f2d0e303e49f2d472106eaad)
@@ -599,4 +599,5 @@
 	int rc;
 	uint8_t *data;
+	unsigned long i;
 
 	data = calloc(cfg->sector_size, 1);
@@ -635,5 +636,18 @@
 
 	rc = block_write_direct(service_id, rootdir_sec, 1, data);
-
+	if (rc != EOK)
+		goto exit;
+
+	/* Fill the content of the sectors not used by the
+	 * root directory with zeroes.
+	 */
+	memset(data, 0, cfg->sector_size);
+	for (i = 1; i < cfg->cluster_size / cfg->sector_size; ++i) {
+		rc = block_write_direct(service_id, rootdir_sec + i, 1, data);
+		if (rc != EOK)
+			goto exit;
+	}
+
+exit:
 	free(data);
 	return rc;