Well, then I'm missing something:
1) I create all partitions on the fresh disk with GPartEd - okay, clear. Having now more space for partition /dev/sda8
2) How and where does the bootloader come into the game? How do I get "something" on the disk that searches for the kernel in /dev/sda6???
Other way around: only performing step 1) does not install a bootloader onto the disk in the way your NAS firmware expects it, right? So where does this come from?