Installing 13.4 on macbook dualboot: grub bootloader installation failed

Installing 13.4 on macbook dualboot: grub bootloader installation failed

Previously I had Ubuntu 12.4 running as second OS on a second boot
partition, installed with bootcamp (no rEFIt). Now I tried to install
Ubuntu 13.4 (amd64) from USB but it failed to install Grub ('bootloader
installation failed') on /dev/sda3 (the fresh Ubuntu partition). Since
there is no further error message, I'm kind of puzzled about this. When I
startup the MacBook and choose "windows" (this name is due to bootcamp and
was that way with 12.4 all the time) from the EFI boot menu, grub "loads"
but says: error, file not found
I googled around and tried a few things:
Running the live-ubuntu and manually installing grub via:
sudo mount /dev/sda3 /mnt
sudo grub-install /dev/sda3 --[b/r]oot-directory="/mnt" --force
... didn't work. Apparently it installed Grub but spit out a lot of
warnings the usage of blocklists and I still couldn't boot.
Installing rEFIt and trying the whole installation over and over again ...
didn't work.
So ... any ideas why grub-install fails? Does the 64bit-version only
supports (u)efi-boot and there are issues due to the old
hardware/firmware?
The setting: Macbook 5,1(late 2008) with OSX 10.8 Ubuntu 13.4 64bit on USB
The disk is healthy (says disk-warrior and diskutil on osx) and the
partition tables are synchronized (rEFIt)
Update: After uninstalling rEFIt, I wasnt able to boot from usb-ununtu
anymore. So I installed rEFInd, run the installation (I think the usb was
booted with efi and not with bios because I didn't see grub before the
installation!) and et voilá, the bootloader installed. BUT: I'm still not
able to boot into the newly installed ubuntu. Same as above: error, file
not found. >grub rescue: ... Did I kill my bootloader??!