First I'd like to say that I've done a lot of search before asking this question. And even if I found some infomations, my question remains.

So here is the situation

A disk with Win7 and Ubuntu in dual boot, but not with grub2. I kept the windows mbr and added grub to a /boot partition, then added an entry in windows mbr with bcd


/dev/sda1 ntfs SYS
/dev/sda2 ntfs WIN
/dev/sda3 EXTENDED
/dev/sda5 ext4 /boot -----> installed the ubuntu mbr here
/dev/sda6 ext4 /
/dev/sda7 ext4 /swap
Free spaces

So when I login, I have bcd showing 2 entries (win7 and the one I have added Ubuntu 12.04). If I choose Ubuntu then I can see the grub2 boot loader of ubuntu. Ok it's fine

Now I'd like to add in the free spaces BT5r3 gnome

So I answer the questions, and I add a new partition ext4 in the free spaces.
BT5 says it will create 2 partitions the one I just created /dev/sda8 and a swap one /dev/sda7 (oups this one already exists... Ok it should be fine)

But my question is about writing the MBR. By default BT5 set it to write the mbr in /dev/sda

I don't want it writes it here. What I have to choose :

1- Write the mbr in /dev/sda5 (my /boot ubuntu partition)
2- Don't write mbr

If I choose the option 2, will it be ok to just update drub in ubuntu (grup-update) ?

I hope you understand the situation. I'm not sure to explain it very well.
Do you have recommandations ?

Thanks a lot by advance for your help