When the appliance product was fully supported then any change in the internal storage itself was not possible as such if your data retention requirements exceeded the capacity your options really came down to get a second appliance and spread the workload or add a SAN attached disk and use it for Backup to Disk (not deduplication)
The appliance product is now completely discontinued - I believe customers with applainces under maintenance do have the option to update to a capacity license and take over reponsibility for the hardware (so in theory you could then change the disk sizes), however if you take up this option I don't think you can legally continue to use the operating system license so you would probably be better off migrating to your own hardware and OS at the same time.
