burncd makes disk that is unmountable

[Dieter]

AMD64 running 6.0
Drive is:
  acd0: DVDR <HL-DT-ST DVDRAM GSA-4160B/A301> at ata0-master UDMA66
Media is CD-RW

Burned a 6.2 disk using:
  burncd data 6.2-RELEASE-amd64-disc1.iso fixate
as suggested in
  http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/creating-cds.html

Seemed to go okay.  Disk boots, but I cannot mount it:

fstab entry:
  /dev/acd0               /cdrom          cd9660  ro,noauto       0       0

Yields:
  g_vfs_done():acd0[READ(offset=32768, length=2048)]error = 5

Tried it with and without "fixate", neither will mount.

Other iso disks (probably burned using NetBSD) mount fine.
UFS DVD+RW disks burned under FreeBSD using growisofs mount fine.

Given the error message, I assume that the block/sector at 32768 isn't
getting written.