Index: sys/fs/msdosfs/msdosfs_fat.c
===================================================================
--- sys/fs/msdosfs/msdosfs_fat.c
+++ sys/fs/msdosfs/msdosfs_fat.c
@@ -908,6 +908,25 @@
 	 * zero.  These represent free clusters.
 	 */
 	pmp->pm_freeclustercount = 0;
+
+	/*
+	 * Check if the FAT is valid and starts in the correct sector.
+	 */
+	fatblock(pmp, FATOFS(pmp, CLUST_FIRST), &bn, &bsize, NULL);
+	error = bread(pmp->pm_devvp, bn, bsize, NOCRED, &bp);
+	if (error) {
+		brelse(bp);
+		return (error);
+	}
+	if ((uint8_t)bp->b_data[0] != pmp->pm_bpb.bpbMedia) {
+		brelse(bp);
+#ifdef MSDOSFS_DEBUG
+		printf("mountmsdosfs(): Media descriptor in BPB"
+		    "does not match FAT ID\n");
+#endif
+		return EINVAL;
+	}
+
 	for (cn = CLUST_FIRST; cn <= pmp->pm_maxcluster; cn++) {
 		byteoffset = FATOFS(pmp, cn);
 		bo = byteoffset % pmp->pm_fatblocksize;