Loosing grub mbr after mounting jffs2 image with loop0 and mtdblock0

[cdlinux at koala.de]

Hello list!

I'm relatively new to linux-mtd and have the following problem:

I'm using 128MB-Cf-Cards with an ext2 and a jffs2 partition.

The image i write to the cf-card has a grub-loader on it that will boot the
jffs2-image on (eg) /dev/hde2...

Now, when I want to change the image-partition, i mount it with

  modprobe loop
  losetup /dev/loop0 imagefile.img
  modprobe mtdcore
  modprobe blkmtd device=/dev/loop0 erasesz=64
  mount -t jffs2 /dev/mtdblock0 /mnt/cfdisk

I then work on the filesystem and unmount everything after the changes are
done.

However, when I write the new image to a cfdisk (dd), I loose the grub info
and booting reports "no valid filesystem" / bootimage...

I suppose, it has to do with the way I mount the jffs2 partition (as I'm
actually mounting the entire image as loopdev/jffs2)...?
I'm unsure about what would be the right way to do this.

Any ideas, on how to proceed working on the 2nd partition of hde as jffs2
w/o loosing the grub mbr?

Thanks for any answers or pointers in the right direction, 
and sorry if this has been asked before, i couldn't find an appropriate 
answer yet..

Greetings
chris