Chainload GRUB
From MEPIS Documentation Wiki
You can install GRUB on root of the device and load it with another boot loader that's installed on MBR by chain loading.
For example if on MBR you have GRUB and you want that GRUB to load another GRUB that's installed in the root of hda1 partition then you have to edit /boot/grub/menu.lst for the MBR GRUB and add these lines:
title MEPIS 7 at sda1 rootnoverify (hd0,0) makeactive chainloader +1
The makeactive command is limited to primary partitions so (hd0,3) which is extended or greater will return invalid device to resolve try
title MEPIS 8 at sda6 rootnoverify (hd0,5) #makeactive chainloader +1
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