Thanks for your kind reply.
I formatted both of the usb and the hhd so that I could try to re-install the ubuntu. As a result, it went worse -- the installation always got stuck in the "purple screen" (with the 'white points' under the ubuntu logo refusing to 'move on'). Well, fine, let's take a alternative. Then I tried to install the 'linuxmint 18 Cinnamon', and the installation always complaint that 'cannot find a medium containing a live file system'. I tried and repeated and searched solutions to these two installations over and over again, but all efforts have been in void.
As a newbie, I know I have much to learn before saying 'enough' to this new world. But now it seems I don't have any chance to gain experience in linux.
Here are more details of the installation environment for your reference:
1. fast boot and secure boot have been turned off;
2. 20GB disk spaces have been allocated in an external hhd for installation (but never able to get applied except for the very first time when successfully installing ubuntu).
3. 16GB USB 3.0 has been used;
4. Processor: Intel Core i7-6700HQ CPU @ 2.60HGz 2.59HGz
5. RAM: 16GB(15.9GB usable)
6. 64-bit, x64-based processor
7. touch support