Good that you have been able to fix the issue at hand. Now, in spite of all
the other paragraphs in this thread, there might actually be a number of real
issues which prevents software from altering the MBR.
So first, for clearance sake, what is an MBR and what is the GPT. Both are
the initial record on a disk, and indicate the following two important
things: 1) how does the disk look like (what are the [primary] partitions)
and if available, where to boot from.
Now GPT and MBR appear to be logically the same, yet they are not. The GPT
uses what is called a protective MBR, sometimes called the GPT protected
partition. Now, this really is in itself an MBR with one primary partition,
the GPT. Now, there are BIOS and/or EFI firmwares which require
authentication to change this part (the MBR or protective MBR).
Still, if you are updating a GPT, the BIOS won't attempt to terminate that
attempt, so happily the partitioner goes. GParted, Parted, FDisk, Ubuiquity
or whatever else is used by the Trisquel installer will also just happily
accept the situation and rewrite the GPT. GRUB 2 at the other end, needing to
write into the (protective) MBR will then fail miserably and booting into
whatever partition will likewise fail.
It might be that your pesky motherboard is, in a desperate attempt of
security, trying to halt you from altering the MBR in the first place.
