luemmel,

Welcome to Justlinux!

If you boot a Linux using "chainloader +1" it will work only if its Grub has been installed in the root partition. In the case of Linux Mint in sdc8 that means /dev/sdc8. Legacy Grub or Grub1 can boot Grub2 this way. In fact any Grub can "chainload" any PC operating system as long as it has its boot loader residing inside the root partition.

Grub2 uses grub.cfg and Grub1 uses menu.lst for configuration.

You can make a Grub2 system installed in its own partition using the Task D2 (with the partition name adjusted for your case) of the Just Booting tips in my signature. If you have used Ext4 in sdc8 then you need a Linux Live CD that can read it.

If you still have a problem then post the content of grub.cfg here and let us know what filing system of sdc8.

As regarding your other booting solution of Mint my explanation would be Grub2 is using different commands to Grub1 so the only way to boot a Grub2 system is by chainloading. Alternatively you can make a Grub2 floppy to boot it manually. I believe in such case the commands would be something like
Code:
set root=(hd0,8)
linux kernel /boot/grub/core.img
boot
The partition numbering system has been changed in Grub2. Also the command "kernel has been replaced by "linux".

Good luck.