The NIOS image was mounted to the VM in order to install the OS. Now it's installed, I want to remove the virtual disk from the VM but I have to unmount it in Ubunutu first.

Not in the same network view. Creating overlapping IP space defeats authoritative IPAM. Perhaps worth looking into: https://docs.infoblox.com/space/nios90/280404336/Configuring+Network+Views