[Yaffs] YAFFS fs images not working the way I expect

I'm trying to dump an image of a YAFFS partition so that I can reload
it later. Things are not working the way I expect. Here is an
example of where I'm unable to copy an image onto a partition and
mount it. Any clues for sale?

1. create a new YAFFS filesystem on mtd partition 2 using a
pregenerated yaffs.dat file, mount it, and verify that the etc and
home directories are listed as expected:

  # cd /path/to/mounted/CF/card
  # /sbin/mkyaffs /dev/mtd/2 /yaffs.dat
  # /bin/mount -n /dev/mtdblock/2 /mnt/flash0 -t yaffs
  # ls /mnt/flash0
  etc         home        lost+found
  #

2. unmount that and dump it to a file:

# umount /mnt/flash0
# /sbin/mkyaffs /dev/mtd/2 /yaffs.dat
argc 3 sh 0 optcnt 1
Erasing and programming NAND
Erasing block at 0x080
Erasing block at 0x0810000
<snipping much Erasage...>
Erasing block at 0x08354000
OK
# dd if=/dev/mtd/2 of=mtd2.img bs=16384
214+0 records in
214+0 records out
#

3. copy zeros onto the partition and verify that we can't mount it
anymore

# dd if=/dev/zero of=/dev/mtd/2 bs=16384 count=214
214+0 records in
214+0 records out
# mount -n /dev/mtdblock/2 /tmp/yaffs -t yaffs
yaffs: dev is 7938 name is "1f:02"
mount: Mounting /dev/mtdblock/2 on /tmp/yaffs failed: Not a directory
#

4. copy the backup image onto the partition, try (and fail) to mount
it, and wonder what I'm doing wrong:

# dd if=mtd2.img of=/dev/mtd/2 bs=16384
214+0 records in
214+0 records out
# /bin/mount -n /dev/mtdblock/2 /mnt/flash0 -t yaffs
yaffs: dev is 7938 name is "1f:02"
mount: Mounting /dev/mtdblock/2 on /tmp/yaffs failed: Not a directory
#