The problem is the EFI does not recognise the bootloader and even though GRUB is installed it is not able to run correctly due to not being able to be read from the MBR.
I found a solution on Ubuntu Forums that hopefully will solve your issue and get your installation working.
SOLUTION 1) GPT partitioning seemed to be a problem for the Ubuntu
installer. I took the SSD disk out of the PC, plugged it to another PC
with a SATA-to-USB adapter and created a normal partition table on it
without any of the non-working GPT crap. I created the partitions as
follows (with cfdisk, gparted also works):
1: FAT32 partition for the EFI bootloader. I set this to 500 MB size
and formatted it with mkfs.vfat. 2: / (root) partition (ext4). 3:
/home partition (ext4).
I use no swap on computers like this that have 16 - 32 GB of RAM.
2) I kept the SSD disk plugged to the other PC with the SATA-to-USB
adapter and mounted the FAT16 EFI partition. I created a folder
efi/grub under it with the command "mkdir -p efi/grub".
3) Now the disk was ready for installation. I put the SSD disk back to
the new PC, booted up Ubuntu installation from the USB disk and
installed it as usual, formatting / and /home as ext4 in the process.
SUCCESS!!! The PC boots in about 10-15 seconds like you'd expect from
an SSD disk. The network card on this motherboard works on Ubuntu
11.10 out-of-the-box as well. 10.04 doesn't seem to recognise it, 11.04 I'm not sure.
My advice to others reading Ubuntu's EFI/UEFI instructions: Do NOT
start recompiling GRUB or other more complicated stuff. It is
completely unnecessary.
The instructions are outdated for 11.10 and just creating the FAT
partition with a folder efi/grub is enough for Ubuntu to automatically
recognise and install the bootloader there. Ubuntu 11.10 already has a
working GRUB for EFI systems, you do NOT need to compile one yourself,
at least not for the Asus EZ.
So, in short: create the partitions on a non-EFI system, mkdir 2
folders and install. If I only knew from the start that it was this
simple.
Reference
Although this references removing the drive from the PC other threads have not removed the drive, used the same method and had success - good luck and let us know how it goes.