TeraByte Unlimited

I used Makedisk to create a bootable USB for IFU_GUI V3.43 .
I can boot into IFU and proceed through the backup process.

When I get to File Access and select File (OS), the next screen offers only:
efi
System Volume Information
C:

How do I mount another USB Drive to save the image to? Does the entire image have to fit on the IFU media?
What is the process to mount network drives?

With IFU, the drive needs to be available to UEFI to be accessible. Some BIOS's handle external drives better than others. In many cases, external drives may not be seen consistently or may be too slow to use. Even internal drives may be much slower via UEFI than other methods.

Use the "File (Direct)" method instead of "File (OS)" to access the drives and see if that works. Otherwise, If you want to use external or network drives you'd be better off using IFL or TBWinRE.