The problem comes from a mistake of mine, not from a system bug. Asking to install grub in /dev/sda instead of sd0 resulted in a correct installation of Ubuntu on an USB esternal drive and grub on it's boot sector.
thanks francesco -- installer crashed upon installing grub on usb drive https://launchpad.net/bugs/77979 -- ubuntu-bugs mailing list [email protected] https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
