Comment 2 for bug 631943

The installer attempts to find the best location for the bootloader. If the installer environment has been booted from a USB disk, it will avoid installing grub to it, installing to the target disk instead. That said, if you're installing to a USB disk, the bootloader will be installed to it as well.

We considered putting the option to select the bootloader on the automatic partitioning page, as we had on the advanced partitioning page, but we ultimately decided against it. The majority of choices in such a drop down box will result in an unbootable system, and we simply don't want to put such a complex and risky decision in front of non-advanced users.

As mentioned, you can still set the bootloader choice by going to the advanced partitioning page. In a later version of Ubuntu, we'll make it so that the partitioning recipe from the automatic page gets applied to the advanced page when you click on the link to the advanced page from the automatic page (currently written as "X smaller partitions are hidden. Use the <link>advanced partitioning tool</link> for more control").