Comment 1 for bug 317085

Thank you for taking the time to report this bug and help to improve Ubuntu.

This is altogether unreproducible here. Using a separate partition for /boot is a common configuration (the system I'm typing from is set up this way), and update-grub has long supported it.

Is your /boot filesystem not mounted at /boot, or is it not listed in /etc/fstab? update-grub calls boot_device=$(find_device "/boot") to determine whether /boot is a separate filesystem, and find_device is a function that looks up the mount in /etc/fstab. The only way I can see this failing is if one of the above two conditions aren't satisfied.