Ubuntu 12.04 does not boot from the disk after installation

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http://askubuntu.com – Installed Ubuntu on HP xw4600 workstation using CD not USB. But after installation it does not boot from hard drive. During installation it showed: The entire disk will be used: /dev/mapper/isw_digjcabegb_Volume0(ext4) 250.1GB Running “grub-install /dev/mapper”… Error pop-up: Unable to install GRUB in /dev/mapper Executing ‘grub-install /dev/mapper’ failed. This is a fatal error. Then another error: Sorry, an error occurred and it was not possible to install the bootloader at the specified location. How would you like to proceed? Choose a different device to install the bootloader (HowTos)