[Littlemaster]

This is possible but if I remember correctly you need a bigger pen drive than 8GB. This is the drive is a bootable one?

The pendrive is 32 GB one, and I have ubuntu on it, that means I can boot to live session. I actually don't know what meant by "Bootable" one.

you can get root in live environment using sudo and then just remount the /cdrom (probably) drive as read write which will enable you to save things that you do as root.

else,
live session with persistent parameter .... or create bootable usb with that set.

else just install ubuntu the normal way on that usb drive.

I will check to use sudo and let you know result. But I tried to install Ubuntu the normal way using the pendrive. But it was giving error. 

What you’re looking for is referred to a live OS with persistent storage.

This guide shows you how to get it done: https://www.howtogeek.com/howto/14912/cr...ash-drive/

That means I need another pendrive, I will try this too.