Invalid Signature with GRUB

A MacBook Pro 7.1 configured for triple booting between MacOS, Linux & Windows failed to load Windows from Grub bootloader, reporting “invalid signature”.

Configured with Refind boot manager and a Hybrid MBR, which initially had the Ext4 Linux partition in the MBR but not the NTFS Windows partition, we attempted to chainloader the Windows boot sector but Grub would return “invalid signature”. The same occurred trying to chainload directly to bootmgr.

To fix the problem the both the EXT & NTFS partitions must have an entry in the hybrid MBR.