How can I reuse old Dorbo drive, dificulty formatting on a Mac

I have a 5N
Just replaced 2 drives (removed a 3TB and a 2TB drive)
I would now like to use these drives as a HD on both a MAC and A PC?
I only have access to a Mac. (I read on a similar forum post, this can be done on a PC relatively easily)
I went to disk utility and ran Erase - MS-DOS (FAT) on the 3TB HD
I ran security options at the second setting and wiped - took hours to complete.
The HD mounts when I connect it to the Mac, however it only shows 800Gb available. It has not seemed to have wiped the whole 3TB.

Any idea how I can format the Drive via my Mac to be used as a HD on both Mac and PC?

I have bought a ORICO toaster USB HD holder which says on Amazon can run HD up to 10TB

ORICO USB 3.0 Hard Drive Docking Station for 2.5 3.5 Inch SATA I/II/III HDD SSD - Support UASP & Up to 10TB

Any help on this issue would be great.

Any ideas how to reformat the old drobo drives so I can get access to all the space, not just 800gb