Index: sys/geom/geom_io.c
===================================================================
--- sys/geom/geom_io.c
+++ sys/geom/geom_io.c
@@ -925,17 +925,25 @@
 	struct bio *bp;
 	void *ptr;
 	int errorc;
+	off_t start_align;
+	off_t end_align;
 
-	KASSERT(length > 0 && length >= cp->provider->sectorsize &&
-	    length <= MAXPHYS, ("g_read_data(): invalid length %jd",
-	    (intmax_t)length));
+	KASSERT(length > 0 && length <= MAXPHYS,
+	    ("g_read_data(): invalid length %jd", (intmax_t)length));
 
+	/* allow unaligned reads */
+	start_align = (offset % cp->provider->sectorsize);
+	end_align = (cp->provider->sectorsize - ((offset + length) %
+	    cp->provider->sectorsize)) % cp->provider->sectorsize;
+
+	/* setup BIO request */
 	bp = g_alloc_bio();
 	bp->bio_cmd = BIO_READ;
 	bp->bio_done = NULL;
-	bp->bio_offset = offset;
-	bp->bio_length = length;
-	ptr = g_malloc(length, M_WAITOK);
+	bp->bio_offset = offset - start_align;
+	bp->bio_length = length + start_align + end_align;
+
+	ptr = g_malloc(bp->bio_length, M_WAITOK);
 	bp->bio_data = ptr;
 	g_io_request(bp, cp);
 	errorc = biowait(bp, "gread");
@@ -945,6 +953,10 @@
 	if (errorc) {
 		g_free(ptr);
 		ptr = NULL;
+	} else {
+		/* check if buffer data should be shifted down */
+		if (start_align != 0)
+			memmove(ptr, (uint8_t *)ptr + start_align, length);
 	}
 	return (ptr);
 }
@@ -979,21 +991,42 @@
 g_write_data(struct g_consumer *cp, off_t offset, void *ptr, off_t length)
 {
 	struct bio *bp;
+	void *fixup;
 	int error;
+	off_t start_align;
+	off_t end_align;
 
-	KASSERT(length > 0 && length >= cp->provider->sectorsize &&
-	    length <= MAXPHYS, ("g_write_data(): invalid length %jd",
-	    (intmax_t)length));
+	KASSERT(length > 0 && length <= MAXPHYS,
+	    ("g_write_data(): invalid length %jd", (intmax_t)length));
 
+	/* allow unaligned writes */
+	start_align = (offset % cp->provider->sectorsize);
+	end_align = (cp->provider->sectorsize - ((offset + length) %
+	    cp->provider->sectorsize)) % cp->provider->sectorsize;
+
+	/* check if we need to do a fixup request */
+	if (start_align != 0 || end_align != 0) {
+		fixup = g_read_data(cp, offset - start_align,
+		    length + start_align + end_align, &error);
+		if (fixup == NULL)
+			return (error);
+		memcpy((uint8_t *)fixup + start_align, ptr, length);
+	} else {
+		fixup = ptr;
+	}
+
+	/* setup BIO request */
 	bp = g_alloc_bio();
 	bp->bio_cmd = BIO_WRITE;
 	bp->bio_done = NULL;
-	bp->bio_offset = offset;
-	bp->bio_length = length;
-	bp->bio_data = ptr;
+	bp->bio_offset = offset - start_align;
+	bp->bio_length = length + start_align + end_align;
+	bp->bio_data = fixup;
 	g_io_request(bp, cp);
 	error = biowait(bp, "gwrite");
 	g_destroy_bio(bp);
+	if (fixup != ptr)
+		g_free(fixup);
 	return (error);
 }
 
Index: sys/geom/part/g_part_mbr.c
===================================================================
--- sys/geom/part/g_part_mbr.c
+++ sys/geom/part/g_part_mbr.c
@@ -59,10 +59,15 @@
 SYSCTL_UINT(_kern_geom_part_mbr, OID_AUTO, enforce_chs,
     CTLFLAG_RWTUN, &enforce_chs, 0, "Enforce alignment to CHS addressing");
 
+static u_int enforce_512bbs = 1;
+SYSCTL_UINT(_kern_geom_part_mbr, OID_AUTO, enforce_512bbs,
+    CTLFLAG_RWTUN, &enforce_512bbs, 0, "Enforce 512 byte block size");
+
 #define	MBRSIZE		512
 
 struct g_part_mbr_table {
 	struct g_part_table	base;
+	off_t		sectorfact;
 	u_char		mbr[MBRSIZE];
 };
 
@@ -295,6 +300,11 @@
 
 	table = (struct g_part_mbr_table *)basetable;
 	le16enc(table->mbr + DOSMAGICOFFSET, DOSMAGIC);
+
+	if (enforce_512bbs)
+		table->sectorfact = pp->sectorsize / MBRSIZE;
+	else
+		table->sectorfact = 1;
 	return (0);
 }
 
@@ -366,6 +376,7 @@
 g_part_mbr_resize(struct g_part_table *basetable,
     struct g_part_entry *baseentry, struct g_part_parms *gpp)
 {
+	struct g_part_mbr_table *table;
 	struct g_part_mbr_entry *entry;
 	struct g_provider *pp;
 	uint32_t size;
@@ -384,6 +395,7 @@
 	if ((g_debugflags & 0x10) == 0 && size < gpp->gpp_size &&
 	    pp->mediasize / pp->sectorsize > size)
 		return (EBUSY);
+	table = (struct g_part_mbr_table *)basetable;
 	entry = (struct g_part_mbr_entry *)baseentry;
 	baseentry->gpe_end = baseentry->gpe_start + size - 1;
 	entry->ent.dp_size = size;
@@ -402,14 +414,16 @@
 }
 
 static int
-g_part_mbr_probe(struct g_part_table *table, struct g_consumer *cp)
+g_part_mbr_probe(struct g_part_table *basetable, struct g_consumer *cp)
 {
+	struct g_part_mbr_table *table;
 	char psn[8];
 	struct g_provider *pp;
 	u_char *buf, *p;
 	int error, index, res, sum;
 	uint16_t magic;
 
+	table = (struct g_part_mbr_table *)basetable;
 	pp = cp->provider;
 
 	/* Sanity-check the provider. */
@@ -418,13 +432,18 @@
 	if (pp->sectorsize > 4096)
 		return (ENXIO);
 
+	if (enforce_512bbs)
+		table->sectorfact = pp->sectorsize / MBRSIZE;
+	else
+		table->sectorfact = 1;
+
 	/* We don't nest under an MBR (see EBR instead). */
 	error = g_getattr("PART::scheme", cp, &psn);
 	if (error == 0 && strcmp(psn, g_part_mbr_scheme.name) == 0)
 		return (ELOOP);
 
 	/* Check that there's a MBR. */
-	buf = g_read_data(cp, 0L, pp->sectorsize, &error);
+	buf = g_read_data(cp, 0L, MBRSIZE, &error);
 	if (buf == NULL)
 		return (error);
 
@@ -477,7 +496,7 @@
 	first = basetable->gpt_sectors;
 	msize = MIN(pp->mediasize / pp->sectorsize, UINT32_MAX);
 
-	buf = g_read_data(cp, 0L, pp->sectorsize, &error);
+	buf = g_read_data(cp, 0L, MBRSIZE, &error);
 	if (buf == NULL)
 		return (error);
 
@@ -494,6 +513,12 @@
 		ent.dp_ecyl = p[7];
 		ent.dp_start = le32dec(p + 8);
 		ent.dp_size = le32dec(p + 12);
+		if (ent.dp_start % table->sectorfact)
+			continue;
+		ent.dp_start /= table->sectorfact;
+		if (ent.dp_size % table->sectorfact)
+			continue;
+		ent.dp_size /= table->sectorfact;
 		if (ent.dp_typ == 0 || ent.dp_typ == DOSPTYP_PMBR)
 			continue;
 		if (ent.dp_start == 0 || ent.dp_size == 0)
@@ -604,8 +629,8 @@
 			p[5] = entry->ent.dp_ehd;
 			p[6] = entry->ent.dp_esect;
 			p[7] = entry->ent.dp_ecyl;
-			le32enc(p + 8, entry->ent.dp_start);
-			le32enc(p + 12, entry->ent.dp_size);
+			le32enc(p + 8, entry->ent.dp_start * table->sectorfact);
+			le32enc(p + 12, entry->ent.dp_size * table->sectorfact);
 		} else
 			bzero(p, DOSPARTSIZE);
 
@@ -613,6 +638,6 @@
 			baseentry = LIST_NEXT(baseentry, gpe_entry);
 	}
 
-	error = g_write_data(cp, 0, table->mbr, cp->provider->sectorsize);
+	error = g_write_data(cp, 0, table->mbr, MBRSIZE);
 	return (error);
 }