I have Windows 7 and Ubuntu 12.04 on internal hard disk of my Dell n5010 and Centos 6.4 on an external usb hard disk.
I am having problem booting from both the internal and the external hdd.

After installation of Centos 6.4 on external hdd, i was unable to boot into my internal hdd if I remove my external hdd.

then I repaired Windows boot manager using EASY BCD. After that, my external USB hdd won't boot and the system would directly boot into windows boot loader even if i have usb hdd connected.

Please give me a solution so that I can boot into the external hdd when it is connected and boot into the internal hdd if external hdd is not connected.

Looks like you need to use the ubuntu dvd/cd to reinstall grub to the internal harddrive. Then before booting into Ubuntu plug in the external hardrive and run update-grub in Ubuntu.