Install grub when booting from live CD or usb
You need to re-install grub on your drive when booting from usb or live CD. This post is made for you, very straight forward step to do so.
Mount your linux root (/) partition
mount /dev/sda4 /mnt
Mount sys,proc,dev into mnt (you might also have to mount your /boot partition if your boot folder is not in the root partition)
mount -t sysfs sysfs /mnt/sys
mount -t proc proc /mnt/proc
mount -o bind /dev /mnt/dev
Note that above "find /boot/grub/stage1" is to find what to put in "root (hd0,3)" and you would use "find /grub/stage1" if /boot is not in your root partition. Also here I installed grub in the MBR with "setup (hd0)" but you could install it on a partition with "setup (hd0,3)".