Debian Installer Stretch RC2 includes a major update to os-prober for finding other operating systems installed on the system. This updated os-prober should fix some "serious bugs" that led to file-system corruption and other problems. The os-prober is used by Debian so that other operating systems can be listed from the bootloader.
This new Debian Installer release for Stretch switches over from Linux 4.8 to 4.9, has a machine database entry for the Pine64+ ARM board, and various other updates and fixes.
More details on Debian Installer Stretch RC2 via the release announcement.