Comment 1 for bug 153933

Triaged to Incomplete. My understanding of GRUB is that it loads the second stage, kernel, and initrd to the desired partition, but it still must modify the MBR to load the first stage. Since you are using a different boot loader (gag), and I am not familiar with it, it's possible that a bad interaction would occur. I don't see a way around this without using GRUB to manage the bootup or perhaps modifying gag's configuration file.

Since this is a rare use case, I don't see ubiquity being modified to detect and handle a different boot loader other than GRUB. Perhaps a few lines of ubiquity code could check for other boot loaders and then exit gracefully.