Comment 56 for bug 576724

I slept on the problem. I think for the time being I shan't add a warning when installing only to a partition, and I'll just keep things simple. We'll see how it goes, and if there's still substantial confusion evident from user reports then we can revisit this.

The current dialog looks something like this. Please note that I've manually hacked it to include a couple of extra filesystems on my system, for the purposes of testing; /mnt and /media/mybook won't be offered in the finished product.

  The grub-pc package is being upgraded. This menu allows you to select
  which devices you'd like grub-install to be automatically run for, if
  any.

  It is recommended that you do this in most situations, to prevent the
  installed GRUB from getting out of sync with other components such as
  grub.cfg or with newer Linux images it will have to load.

  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 (80026 MB, FUJITSU_MHW2080BJ_G2)
    [ ] - /dev/sda5 (50001 MB, /)
    [ ] - /dev/sda7 (5346 MB, /mnt)
    [ ] /dev/sdb (1000204 MB, My_Book_AV_1020)
    [ ] - /dev/sdb1 (1000203 MB, /media/mybook)

  <Ok>

There may still be some confusion, but at least now if somebody selects all the checkboxes then it's unlikely to be harmful. People with specialised boot loaders that they need to avoid overwriting will likely understand the terminology well enough to be able to select something appropriate.