[Trisquel-users] How can I make a master boot record without resorting to Windows ?

[amenex]

Wishing to install Trisquel 7 onto a clean hard disk, I cleaned out  
everything and created an ext4 partition, but the Trisquel 7 Live DVD insists  
that there is "no defined root file system" and suggests that I "correct this  
from the partitioning menu."

What I've done, of course, is remove the MBR or Master Boot Record.  Now the  
Internet insists that every operating system has (requires ?) a MBR ... but  
is that always a Microsoft MBR ? It seems so, as I haven't found a recipe for  
acquiring/creating a MBR that doesn't involve using a Windows source or  
backup.

Surely there is a GNU/Linux MBR facility for creating a MBR on a clean disk.  
Is that GRUB ? Are GRUB and MBR two terms for the same thing, or does GRUB  
depend on the preexistence of a MBR ?

Searching the Add/Remove Applications menu in Trisquel for "GRUB" reveals no  
matching applications.

I'm doing this to a USB-attached hard drive, not to the main hard drive in  
the computer, as I'm trying to make a Trisquel 7 installation into another  
computer.

What I've tried so far is to attempt the install of Trisquel 7 from the  
LiveDVD to the USB-attached hard drive, which always ends with the  
above-quoted "no root file system defined" complaint ... whether I mount the  
hard drive or not, and whether there's any file already in it or not.

Please point me to a coherent Trisquel (i.e., rational) explanation of the  
process whereby I can put a recognized master boot record onto that hard  
drive.