I used the commands you've suggested to check the install directory for the Grub2 bootloader installed on my system (the drive ata-WDC_WD6400[etc.]), and it seems that it's installed on the Ubuntu partition (/dev/sda10), as you can see by the output below.
sudo debconf-show grub-pc
sudo grub-probe -t device /boot/grub
In fact, Ubuntu was my first Linux distro, so it matches with what I thought it would be, but when I do the other command you suggested (sudo dpkg-reconfigure grub-pc), so that Grub2 remembers on which drive it should reinstall, drive sda appears and the partition sda10. In that case, I should choose the partition sda10, correct? Otherwise nothing will change, or am I wrong?
* grub-pc/install_devices: /dev/disk/by-id/ata-WDC_WD6400BEVT-80A0RT0_WD-WXD1A80X7152
grub2/kfreebsd_cmdline_default: quiet splash
* grub2/linux_cmdline_default: quiet splash