Failed update

If you see a screen like this:

 ┌──────────────────────────────────────┤ Configuring grub-pc ├───────────────────────────────────────┐
 │ The GRUB boot loader was previously installed to a disk that is no longer present, or whose        │
 │ unique identifier has changed for some reason. It is important to make sure that the installed     │
 │ GRUB core image stays in sync with GRUB modules and grub.cfg. Please check again to make sure      │
 │ that GRUB is written to the appropriate boot devices.                                              │
 │                                                                                                    │
 │ If you're unsure which drive is designated as boot drive by your BIOS, it is often a good idea to  │
 │ install GRUB to all of them.                                                                       │
 │                                                                                                    │
 │ Note: it is possible to install GRUB to partition boot records as well, and some appropriate       │
 │ partitions are offered here. However, this forces GRUB to use the blocklist mechanism, which       │
 │ makes it less reliable, and therefore is not recommended.                                          │
 │                                                                                                    │
 │ GRUB install devices:                                                                              │
 │                                                                                                    │
 │    [ ] /dev/sda (53687 MB; VBOX_HARDDISK)                                                          │
 │    [ ] - /dev/sda1 (999 MB; /boot)                                                                 │
 │    [ ] /dev/dm-0 (9999 MB; vg00-lvroot)                                                            │
 │                                                                                                    │
 │                                                                                                    │
 │                                               <Ok>                                                 │
 │                                                                                                    │
 └────────────────────────────────────────────────────────────────────────────────────────────────────┘

The right option id the first one /dev/sda (53687 MB; VBOX_HARDDISK), because it is the hard disk in which the /root partition is installed.

Installing grub in a partition (e.g. choosing the second option) is advised against:

(http://askubuntu.com/questions/19704/bad-idea-warning-when-trying-to-recover-grub-after-windows-removed-it)

    - The master boot record, MBR, is the first 512 byte 'sector' of any partitioned hard disk.
    - The BIOS searches this sector when trying to find a device it can boot from.
    - In the GNU/Linux system, your hard disks are all named /dev/sd[x], where x are consecutive letters. E.g.: /dev/sda and /dev/sdb. The first partition on the first disk is called /dev/sda1. The fourth partition on the seventh hard disk is called /dev/sdg4, and so on. At the very least, you'll have one partition.
    - These partitions are not searched for bootable data by the bios. As a result, grub-install [...] /dev/sda7 will issue a warning. You can install it there, but in almost any case, you shouldn't.