I tried to install from the installation media . I put the install cd and then tried for 'expert' and select /dev/sda instead of /dev/hda as the media to install on. But at the end my machine cannot be booted from the usb disk. Can you please let me know how can I make it as bootable disk?
CentOS mailing list CentOS@centos.org http://lists.centos.org/mailman/listinfo/centos
Done that way I'd be surprised if you had a bootable machine.... Unless you mean tried to install within the vbox session... /dev/sda would most likely wipe out your internal hard disk (where you said you have windows installed).
What are you actually trying to accomplish? You should note that grub will not always play nice being installed on/running from an external usb drive if you are intending to use it as a portable distro... no guarantees that the device.map will be valid from one machine to the next...