BOOT PROBLEM

Hi and welcome :)

1- https://trisquel.info/en/search/node/GRUB

I'm sure if you read in there you'll find an answer.

2- startx

/dev/sdb5 does not exist

You apparently put part of the file hierarchy on second disk that is not plugged anymore! What is the content of your /etc/fstab ('cat /etc/fstab' should tell but you can use a live system too).

I actually looked at the attached picture after answering the following (that does not apply to you: you have a more serious issue):

The NetInstall is minimal Nothing graphical. If you actually want Trisquel's default interface, execute:
$ sudo apt-get install trisquel trisquel-recommended
To only get, a basic set of applications, only install "trisquel". If you prefer Trisquel mini, the two packages are "trisquel-mini" and "trisquel-mini-recommended", etc.

If you prefer to customize your choice of applications, you can, instead, install them one by one (you need to know their names). When you will install the first graphical application, the X server will come as a dependency and the 'startx' will then be available. If you want to boot in the graphical session and log in there, choose a display manager: LightDM is Trisquel's default (it comes in dependency of "trisquel") but I personally prefer GDM.