What are the best ways to fix MBR and GPT , and what are the best commands to do so ?

if Red hat/Centos/Fedora

just boot witht the rescue CD

then execute the following commands

# chroot /mnt/sysimage

Re-install Grub

ex. if boot partation on (/dev/sda):

# grub-install /dev/sda

first, MBR is different than GPT, second what OS you working on?

Hello, to fix MBR it will be better to boot from live cd image and use HDD regenerator or use utils like "fix MBR" in DOS,