When expanding Internal backup storage in the Unitrends Backup virtual appliance after adding a new attached disk to the backup virtual machine, the disk shows as already added to internal storage prior to clicking "Add".
This could be caused if the initial 100GB virtual disk that is created by default during the installation process via the installation EXE or OVA has been expanded in the virtual environment instead of attaching a second virtual disk and adding into internal storage in the Unitrends web interface. Unitrends does not support expanding the size of virtual disks on the virtual host. You may only add additional virtual drives and add to internal storage in the Unitrends web interface.
Unitrends strongly recommends following documented instructions when adding backup storage.