The solution is to partition it beforehand using something like GParted. Using GParted I created two partitions. First is a small 100mb FAT32, and the second is EXT4 covering the rest of the drive. Then you run the installer like shown above, choose the second partition(Don't format it!) and then say yes when it asks to install GRUB(skip EFI). GRUB installs to the first partition while Android-x86 installs to the second.