Is this a brand new 8TB drive you just got or was it previously working to see all 8TB?
One of the easiest ways to try to mess with this is actually to try and erase it on a Windows PC to see if that recognizes all 8TB if you have no data on there. If so, then put it back to your iMac and let it erase it and it will probably see all 8TB at that point. Doing it this way saves a lot of painful troubleshooting in a Terminal command window.
If you've never seen the 8TB fully on this (new?) drive then you'll want to check out Seagate's support, including their support software for the Mac. They do have a firmware updater for the Mac:
https://www.seagate.com/support/downloads/item/seagate-slim-mac-firmware-master-dl/
They also have some very advanced low-level support software on their website for free, but most of that is Windows-only if you do have access to a Windows machine.
If it's new I would encourage you to contact them. They are strongly motivated to get you not to return it to the store, or not to send it in for warranty service and are usually pretty good at working with people. Again though, doing it with a Windows PC is better as it opens up more of their tools and eliminates anything particular with your Mac as a possibility.
