I have used Boot-Repair-Disk a lot. It has saved my computer systems many times, since I do a lot of experimenting on them. The title program, boot-repair, can often fix the boot procedure on Linux systems if that has been messed up. This happens to me more than it should since my computers typically can boot Windows and one or more flavors of Linux. Altering one OS sometimes causes one of the others to be unbootable.

Other programs on the disk are also useful. For example, OS-Uninstaller does exactly what the name suggests. GParted lets you add, move, delete, and resize disk partitions, which I often need to do in preparation for adding a new OS (You need to be careful with this, especially for bootable partitions). The whole disk is actually a standalone Linux system, so with its Linux terminal you can access and work with any of the drives on your system.