Index: sbin/fsck_msdosfs/fat.c
===================================================================
--- sbin/fsck_msdosfs/fat.c
+++ sbin/fsck_msdosfs/fat.c
@@ -33,6 +33,8 @@
   "$FreeBSD$";
 #endif /* not lint */
 
+#include <sys/endian.h>
+
 #include <stdlib.h>
 #include <string.h>
 #include <ctype.h>
@@ -623,30 +625,54 @@
 		case CLUST32_MASK:
 			if (fat[cl].next == CLUST_FREE)
 				boot->NumFree++;
-			*p++ = (u_char)fat[cl].next;
-			*p++ = (u_char)(fat[cl].next >> 8);
-			*p++ = (u_char)(fat[cl].next >> 16);
-			*p &= 0xf0;
-			*p++ |= (fat[cl].next >> 24)&0x0f;
+			le32enc(p, fat[cl].next | 0xF0000000);
+			p += 4;
 			break;
 		case CLUST16_MASK:
 			if (fat[cl].next == CLUST_FREE)
 				boot->NumFree++;
-			*p++ = (u_char)fat[cl].next;
-			*p++ = (u_char)(fat[cl].next >> 8);
+			le16enc(p, fat[cl].next);
+			p += 2;
 			break;
 		default:
 			if (fat[cl].next == CLUST_FREE)
 				boot->NumFree++;
-			*p++ = (u_char)fat[cl].next;
-			*p = (u_char)((fat[cl].next >> 8) & 0xf);
+			le16enc(p, fat[cl].next & 0xfff);
+			p++;
 			cl++;
 			if (cl >= boot->NumClusters)
 				break;
 			if (fat[cl].next == CLUST_FREE)
 				boot->NumFree++;
-			*p++ |= (u_char)(fat[cl + 1].next << 4);
-			*p++ = (u_char)(fat[cl + 1].next >> 4);
+			/*
+			 * Pack the 2nd 12-bit cluster, if there is one.
+			 *
+			 * Currently:
+			 *       [ cl-1:next ][   zeros   ]
+			 * Bits: mmmmmmmm mmmm0000 00000000
+			 *       ^        ^        ^
+			 *       |        |        (p+1)
+			 *       |        p; high 4b of cl-1:next
+			 *       (p-1); low 8b of cl-1:next
+			 *
+			 * We want to store the 12 bit cl:next at p + half a
+			 * byte.  But we can only point to bytes in C.
+			 *
+			 * Instead, create a full 16 bit value to store at p.
+			 * It is composed of the 12 bit cl:next pointer,
+			 * appended with the existing upper 4 bits from the
+			 * cl-1:next pointer at *p.  Store it over p.
+			 */
+			le16enc(p, ((fat[cl].next & 0xfff) << 4) | *p);
+			p += 2;
+			/*
+			 * The result is:
+			 *
+			 *       [ cl-1:next ][  cl:next  ]
+			 * Bits: mmmmmmmm mmmmCCCC CCCCCCCC
+			 *                                  ^
+			 *                                  p
+			 */
 			break;
 		}
 	}