My iMac HD is not showing all capacity My iMac has a 2 TB Fusion Drive. Our PC IT guy seems to screwed it up. Now I only have 1 TB of usable space. I tried formatting, repartitioning, but nothing works to gain that 1 TB back.

I don't need the data in it as this is a new iMac.
It seems that you have only one partition of 1 TB.
What you need to do is :


Select "Fusion Drive"
Click on "Partition" button
Set a partition of 2 TB


I don't have my MacBook so I can't make you some screenshots, but setting new partition should show you your all disk.

But on your second screenshot, I can see that the Untitled part is 2.12 TB. Did you tried to proceed to the install of macOS and then see if everything is all good ?