diff --git a/sys/dev/scsipi/scsipi_base.c b/sys/dev/scsipi/scsipi_base.c
index 970c6f011219..a7c4f23c9047 100644
--- a/sys/dev/scsipi/scsipi_base.c
+++ b/sys/dev/scsipi/scsipi_base.c
@@ -1,4 +1,4 @@
-/*	$NetBSD: scsipi_base.c,v 1.139 2006/10/12 01:31:57 christos Exp $	*/
+/*	$NetBSD: scsipi_base.c,v 1.140 2006/10/20 07:11:50 scw Exp $	*/
 
 /*-
  * Copyright (c) 1998, 1999, 2000, 2002, 2003, 2004 The NetBSD Foundation, Inc.
@@ -38,7 +38,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: scsipi_base.c,v 1.139 2006/10/12 01:31:57 christos Exp $");
+__KERNEL_RCSID(0, "$NetBSD: scsipi_base.c,v 1.140 2006/10/20 07:11:50 scw Exp $");
 
 #include "opt_scsi.h"
 
@@ -1020,13 +1020,46 @@ scsipi_interpret_sense(struct scsipi_xfer *xs)
 	}
 }
 
+/*
+ * scsipi_validate_secsize:
+ *
+ *	Validate the sector size reported by READ_CAPACITY_1[06].
+ *	Use the supplied default if the reported size looks wrong.
+ */
+static int
+scsipi_validate_secsize(struct scsipi_periph *periph, const char *opcode,
+    int raw_len, int def_len)
+{
+
+	switch (raw_len) {
+	case 256:
+	case 512:
+	case 1024:
+	case 2048:
+	case 4096:
+		break;
+
+	default:
+		scsipi_printaddr(periph);
+		printf("%s returned %s sector size: 0x%x. Defaulting to %d "
+		    "bytes.\n", opcode, (raw_len ^ (1 << (ffs(raw_len) - 1))) ?
+		    "preposterous" : "unsupported", raw_len, def_len);
+		/*FALLTHROUGH*/
+	case 0:
+		raw_len = def_len;
+		break;
+	}
+
+	return (raw_len);
+}
+
 /*
  * scsipi_size:
  *
  *	Find out from the device what its capacity is.
  */
 u_int64_t
-scsipi_size(struct scsipi_periph *periph, int *secsize, int flags)
+scsipi_size(struct scsipi_periph *periph, int *secsize, int defsize, int flags)
 {
 	union {
 		struct scsipi_read_capacity_10 cmd;
@@ -1050,8 +1083,11 @@ scsipi_size(struct scsipi_periph *periph, int *secsize, int flags)
 		return (0);
 
 	if (_4btol(data.data.addr) != 0xffffffff) {
-		if (secsize)
-			*secsize = _4btol(data.data.length);
+		if (secsize) {
+			*secsize = scsipi_validate_secsize(periph,
+			    "READ_CAPACITY_10", _4btol(data.data.length),
+			    defsize);
+		}
 		return (_4btol(data.data.addr) + 1);
 	}
 
@@ -1071,8 +1107,10 @@ scsipi_size(struct scsipi_periph *periph, int *secsize, int flags)
 	    flags | XS_CTL_DATA_IN | XS_CTL_DATA_ONSTACK | XS_CTL_SILENT) != 0)
 		return (0);
 
-	if (secsize)
-		*secsize = _4btol(data.data16.length);
+	if (secsize) {
+		*secsize = scsipi_validate_secsize(periph, "READ_CAPACITY_16",
+		    _4btol(data.data16.length), defsize);
+	}
 	return (_8btol(data.data16.addr) + 1);
 }
 
diff --git a/sys/dev/scsipi/scsipiconf.h b/sys/dev/scsipi/scsipiconf.h
index c026c21591dc..80ea6b1ef8af 100644
--- a/sys/dev/scsipi/scsipiconf.h
+++ b/sys/dev/scsipi/scsipiconf.h
@@ -1,4 +1,4 @@
-/*	$NetBSD: scsipiconf.h,v 1.104 2006/10/09 21:29:14 scw Exp $	*/
+/*	$NetBSD: scsipiconf.h,v 1.105 2006/10/20 07:11:50 scw Exp $	*/
 
 /*-
  * Copyright (c) 1998, 1999, 2000, 2004 The NetBSD Foundation, Inc.
@@ -638,7 +638,7 @@ const void *scsipi_inqmatch(struct scsipi_inquiry_pattern *, const void *,
 const char *scsipi_dtype(int);
 void	scsipi_strvis(u_char *, int, const u_char *, int);
 int	scsipi_execute_xs(struct scsipi_xfer *);
-u_int64_t scsipi_size(struct scsipi_periph *, int *, int);
+u_int64_t scsipi_size(struct scsipi_periph *, int *, int, int);
 int	scsipi_test_unit_ready(struct scsipi_periph *, int);
 int	scsipi_prevent(struct scsipi_periph *, int, int);
 int	scsipi_inquire(struct scsipi_periph *,
diff --git a/sys/dev/scsipi/sd.c b/sys/dev/scsipi/sd.c
index d5336181288b..5b41db63a994 100644
--- a/sys/dev/scsipi/sd.c
+++ b/sys/dev/scsipi/sd.c
@@ -1,4 +1,4 @@
-/*	$NetBSD: sd.c,v 1.252 2006/10/12 01:31:57 christos Exp $	*/
+/*	$NetBSD: sd.c,v 1.253 2006/10/20 07:11:50 scw Exp $	*/
 
 /*-
  * Copyright (c) 1998, 2003, 2004 The NetBSD Foundation, Inc.
@@ -54,7 +54,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: sd.c,v 1.252 2006/10/12 01:31:57 christos Exp $");
+__KERNEL_RCSID(0, "$NetBSD: sd.c,v 1.253 2006/10/20 07:11:50 scw Exp $");
 
 #include "opt_scsi.h"
 #include "rnd.h"
@@ -1661,7 +1661,7 @@ sd_get_simplifiedparms(struct sd_softc *sd, struct disk_parms *dp, int flags)
 	 * XXX probably differs for removable media
 	 */
 	dp->blksize = 512;
-	if ((sectors = scsipi_size(sd->sc_periph, &blksize, flags)) == 0)
+	if ((sectors = scsipi_size(sd->sc_periph, &blksize, 512, flags)) == 0)
 		return (SDGP_RESULT_OFFLINE);		/* XXX? */
 
 	error = scsipi_mode_sense(sd->sc_periph, SMS_DBD, 6,
@@ -1673,7 +1673,7 @@ sd_get_simplifiedparms(struct sd_softc *sd, struct disk_parms *dp, int flags)
 
 	dp->blksize = _2btol(scsipi_sense.lbs);
 	if (dp->blksize == 0)
-		dp->blksize = blksize ? blksize : 512;
+		dp->blksize = blksize;
 
 	/*
 	 * Create a pseudo-geometry.
@@ -1707,7 +1707,8 @@ sd_get_capacity(struct sd_softc *sd, struct disk_parms *dp, int flags)
 	u_int8_t *p;
 #endif
 
-	dp->disksize = sectors = scsipi_size(sd->sc_periph, &blksize, flags);
+	dp->disksize = sectors = scsipi_size(sd->sc_periph, &blksize, 512,
+	    flags);
 	if (sectors == 0) {
 		struct scsipi_read_format_capacities cmd;
 		struct {
@@ -1762,7 +1763,7 @@ printf("rfc result:"); for (i = sizeof(struct scsipi_capacity_list_header) + dat
 		memset(&scsipi_sense, 0, sizeof(scsipi_sense));
 		error = sd_mode_sense(sd, 0, &scsipi_sense,
 		    sizeof(scsipi_sense.blk_desc), 0, flags | XS_CTL_SILENT, &big);
-		dp->blksize = blksize ? blksize : 512;
+		dp->blksize = blksize;
 		if (!error) {
 			if (big) {
 				bdesc = (void *)(&scsipi_sense.header.big + 1);
@@ -1781,7 +1782,7 @@ printf("page 0 ok\n");
 			if (bsize >= 8) {
 				dp->blksize = _3btol(bdesc->blklen);
 				if (dp->blksize == 0)
-					dp->blksize = blksize ? blksize : 512;
+					dp->blksize = blksize;
 			}
 		}
 	}