Debian installation on recent MacBook Pro: "Cannot read `/grub/core.img' correctly"

[Johannes Graumann <johannes_graumann@web.de>]

Dear all,

I'm struggling to install Debian Testing alongside OSX (Lion; chainloading 
rEFIt/grub2) on my recent MacBook Pro. rEFIt is installed and I shrunk OSX 
(using Bootcamp) to 200 of the 750 available gigs. Whenever I go through 
debian's installation process and partition the available 550g with a 
"/boot" ext2 partition directly following the 2 present OSX partitons, grub2 
fails to install into that boot partition (/dev/sdb4) with the error "Cannot 
read `/grub/core.img' correctly". I have tried MANY proposals found through 
google ... 
1) I installed without a bootloader, resynced the MBR using rEFIt and tried 
to install grub via a rescue disk ...
2) I installed the core system, folloed it with the installation of gptsync, 
synced the MBR and tried to install grub2 ...
Is there anything else I could try to get this to work?

Thanks for any hints, Joh