This article explains that there is no limit to the amount of storage you can add to your Unitrends Free appliance.
There is no limit to the amount of storage that can be added to your Unitrends Free appliance. You can use as much storage as you want to protect up to 1 TB of unique virtual machine data.
Unitrends Free supports attached virtual disk storage, and requires a minimum of 128 GB of backup storage. Additionally, we recommend following your hypervisor's requirements and limitations.
In ESXi versions 5.0 and 5.1, VMware has a maximum virtual disk size of 2 TB. However, ESXi version 5.5 supports larger virtual disks on VMFS-5 datastores. Fore more information, see VMware's Maximum Storage Specifications and Support for virtual machine disks larger than 2 TB in VMware ESXi 5.5.
By default, Hyper-V on Windows Server 2012 and 2012 R2 uses VHDX virtual disks, which have a maximum size of 64 TB. For more information, see Hyper-V Scalability.