Come on guys, Grub2 can boot to the end of a 2TB disk. I tried it on both MSdos and gpt disks with the maximum 128 partitions.
Grub1 may have a problem if asked to boot a partition at about 1.2TB location where it will report "Number of cylinders exceeds that supported by the Bios".
500Gb is fine for both Grub.
If you get a Grubrescue then the partition holding core.img could not be linked to the MBR.
The normal cure
(1) Boot up the Ubuntu as a Live CD and check the partition where it has been installed by command
(2) Say the partition is /dev/sda1 then make a mounting point, mount /dev/sda1 and do a grub-install
mount /dev/sda1 /mnt/sda1
mount --bind /dev /mnt/sda1/dev
grub-install --root-directory=/mnt/sda1 /dev/sda
The sda1 is the correct partition if it has Grub2 system files containing many files including core.img