Fast to the point..
I have installed on my dystem a copy of Fedora 15 and Ubuntu 11.04. I first installed ubuntu on /dev/sda5 and grub2 was instaled at the begining of /dev/sda. Next i installed Fedora on /dev/sda6 and following a few recomendations i installed his boot system at the begining of /dev/sda6. After this, grub couldnt show me the information of my Fedora installation, but i entered ubuntu and applied update-grub, and when i restarted i could enter to Fedora. The problem is that when i updated my Fedora distro from 2.6.38.6-26.rc1.fc15 to 2.6.40-4.fc15 and i applied update-grub on ubuntu after that. At the moment that i want to run the new kernel on grub it appears this Kernel Panic. I dont know what to do, since i found a lot of information about this problem but none applied to this case specificly. If you need more details just ask.
Update:
This are the entries in the file /boot/grub/grub.cfg which is created after running update-grub.
### BEGIN /etc/grub.d/30_os-prober ###
menuentry "Fedora (2.6.40-4.fc15.x86_64) (on /dev/sda6)" --class gnu-linux --class gnu --class os {
insmod part_msdos
insmod ext2
set root='(/dev/sda,msdos6)'
search --no-floppy --fs-uuid --set=root 459a7d80-cc8f-4ae0-89cf-49593a9221e2
linux /boot/vmlinuz-2.6.40-4.fc15.x86_64 ro root=UUID=459a7d80-cc8f-4ae0-89cf-49593a9221e2 rd_NO_LUKS rd_NO_LVM rd_NO_MD rd_NO_DM LANG=en_US.UTF-8 SYSFONT=latarcyrheb-sun16 KEYTABLE=la-latin1 rhgb quiet
}
menuentry "Fedora (2.6.38.6-26.rc1.fc15.x86_64) (on /dev/sda6)" --class gnu-linux --class gnu --class os {
insmod part_msdos
insmod ext2
set root='(/dev/sda,msdos6)'
search --no-floppy --fs-uuid --set=root 459a7d80-cc8f-4ae0-89cf-49593a9221e2
linux /boot/vmlinuz-2.6.38.6-26.rc1.fc15.x86_64 ro root=UUID=459a7d80-cc8f-4ae0-89cf-49593a9221e2 rd_NO_LUKS rd_NO_LVM rd_NO_MD rd_NO_DM LANG=en_US.UTF-8 SYSFONT=latarcyrheb-sun16 KEYTABLE=la-latin1 rhgb quiet
initrd /boot/initramfs-2.6.38.6-26.rc1.fc15.x86_64.img
}
The kernel that's not working is the first one. The only difference i see is that the working kernel has an initrd instruction. I tried to replicate this in the non-working kernel, changing the kernerl value but it didn't work. I hope this could be of some help.
Regards.