Super Grub Disk is a indispensible tool if you have installed GRUB to manage multi-boot on your system. After modifications to the disks or partitions, you’ll often find GRUB complaining about errors (like the common 15 or 17). Often times you just need to modify a few parameters to GRUB like the device or partition identifiers. I have had this happened to me multiple times in the past, and have had to search various forums for hints. By far the most safe and simple solution comes from Super Grub Disk. Check it out. A must-have for system admins.