@DCSOTech OK make sure I understand, you have a 6TB drive that already has FOG images on it and you want to preserve your image files?
If the drive is already connected to your fog server (but not necessarily ubuntu) key in this command and put the output here.
lsblk
This command will show block devices on the computer (think hard drives). From that output we can give you the next steps.
Also one question to avoid many turns on questions. Is your current fog server a new fog server without any captured images, where you want to bring in the new 6TB drive before you start capturing images? (its important to know the approach we will take to connect the drive if we need to save what is currently on the fog server).