I have a Mid 2007 iMac, I've installed rEfind on the mac and the Linux Mint (Cinnamon 17.3) installer on an USB thumb drive .

rEfind sees the USB thumb drive, sees grub installed on the key, I boot from it and start to install Linux Mint, at a certain point I get this error: 'the 'grub-efi-amd64-signed' package failed to install into /target/'.

I manually set the partition table:  

sda4 / k gb   

sda5 swap 4096 mb

The iMac has a Core 2 Duo 2.0 Ghz, 3 GB RAM, and a NVDIA card.

I'll try again using the last UNebootin version, ad some different Linux distro (Xubuntu, Ubuntu, Lubuntu), but I'd like to know what issue could I have.