I think you are confusing yourself and me.
In the Post #38 my suggestion was
Partition only one disk, install all the software in it, use this command to mirror hda to hdb in one operation
This mean you put everything in hda first.
Now it seems that some of the partitions in hda were empty and unformatted. Logically the mirrored ones in hdb must also be empty and unformatted. That you can understand it now.
If you are not sure that source partitions hda2 and hda6 are empty you can mount them and see for yourself.
If you have any data in hda then they all should be in hdb now, perhaps not in hdb2 and hdb6.
In a nutshell
hdb1 is now a mirror image of your hda1
hdb2 is now a mirror image of your hdb2
and so on
Unless since dd you have introduce additional data in hdb2 and hdb6. If this is the case just format the two, mount it and copy the files across by drag and drop, tar or dd. Since you have made hdb2 identical in size to hda2 you can now do
dd if=dev/hda2 of=/dev/hdb2 bs=32256
to mirror the content of hda2 into hdb2, same for hda6 into hdb6, got it ?
The booting sequence is the BIOS will boot the first disk specified in the boooting queue. Unless you change it the first bootable should be hda. Therefore the Grub in hda will take control. If the hdb is replaced by your Debian disk then Debian will not boot at all. You can boot it only with a Grub floppy or amending the /boot/grub/menu.lst in Mandriva.
If you are transfering personal data from Debian disk (new hdb) to Mandriva disk (hda) it should be a drag and drop exercise, so just mount Debian's root and goto Mandriva desktop to do the file copying.