grub2 can't boot int oother installations / no such device error

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http://forums.fedoraforum.org – Hi, I freshly installed F17 on a SSD (/dev/sda1) which worked fine. Next I installed Ubuntu on /dev/sda3, my intention was to create a multiboot system. After rebooting grub failed with an error message "unknown filesystem". Using the F17 CD I could restore grub2 to boot into F17 again. My problem: within F17 I created a new grub.conf by calling grub2-mkconfig. The second system (Ubuntu) was detected correctly, and a grub2-entry with the correct UUID of /dev/sda3 was added to grub.conf. But now when I try to boot into Ubuntu, after selecting the Ubuntu-menuentry, I get follo (HowTos)