Are you backing up the entire Server B, i.e. with the operating system, system state, etc.? If you are, then BE would have created a .dr for Server B. With this .dr, you can then use the IDR wizard to create the IDR disk for restoring Server B.
However, if you are just backing up Exchange data and files from Server B, but not the operating system, then there is no need to create the IDR disk. To restore Server B, you would need to re-install the OS and the applications. Then use BE to restore the Exchange data and files.
If this answers your query, please mark this as the solution.