Thank you for your help.
No Bootcamp didn't build the windows installer, i have a Microsoft Windows 10 USB stick.
No, the 3TB drive wasn't attached when i first tried to install windows.
Here is the output for diskutil apfs list
Last login: Mon Mar 18 18:31:21 on ttys000
Anthonys-iMac:~ diskutil apfs list
APFS Container (1 found)
|
+-- Container disk2 546F2AED-E676-4A31-8A73-C7A9DA10F823
====================================================
APFS Container Reference: disk2
Size (Capacity Ceiling): 878524215296 B (878.5 GB)
Capacity In Use By Volumes: 179478732800 B (179.5 GB) (20.4% used)
Capacity Not Allocated: 699045482496 B (699.0 GB) (79.6% free)
|
+-< Physical Store disk0s2 CA074651-2BD2-4E01-A8E4-E468A83ABE3D
| -----------------------------------------------------------
| APFS Physical Store Disk: disk0s2
| Size: 878524215296 B (878.5 GB)
|
+-> Volume disk2s1 DC9AF507-B548-3515-B739-CEB818297ED0
| ---------------------------------------------------
| APFS Volume Disk (Role): disk2s1 (No specific role)
| Name: Macintosh HD (Case-insensitive)
| Mount Point: /
| Capacity Consumed: 176570867712 B (176.6 GB)
| FileVault: No
|
+-> Volume disk2s2 E008F454-BD05-478B-9E24-E5FF21611C1C
| ---------------------------------------------------
| APFS Volume Disk (Role): disk2s2 (Preboot)
| Name: Preboot (Case-insensitive)
| Mount Point: Not Mounted
| Capacity Consumed: 46886912 B (46.9 MB)
| FileVault: No
|
+-> Volume disk2s3 3508320D-3A45-4E5F-B899-322051F00EB5
| ---------------------------------------------------
| APFS Volume Disk (Role): disk2s3 (Recovery)
| Name: Recovery (Case-insensitive)
| Mount Point: /Volumes/Recovery
| Capacity Consumed: 516952064 B (517.0 MB)
| FileVault: No
|
+-> Volume disk2s4 6A10C4DD-3D0A-4B5B-8EB3-E765FF2E2A44
---------------------------------------------------
APFS Volume Disk (Role): disk2s4 (VM)
Name: VM (Case-insensitive)
Mount Point: Not Mounted
Capacity Consumed: 2147504128 B (2.1 GB)
FileVault: No