Multibooting is a process that enables users to install multiple operating systems on a single hard drive. This is achieved by creating separate partitions for each OS, and then using a bootloader to manage the boot process. The bootloader presents a menu at startup, allowing users to select which OS to boot into.
A failed multiboot HDD is almost always due to poor partitioning. Here is the for a 1TB drive.
The most modern method; you simply drag and drop ISO files onto the drive. Easy2Boot (E2B): A highly compatible legacy/UEFI script-based loader. Grub4Dos / GRUB2: