If you retain the 500GB as the backup - would that not mean you would have to go through partitioning again in the event of a failure? I went through the pain of this with mine and went as far as finding the OEM Hitachi Travelstar HDD to keep the system "original". I'm not sure if these Hitachi disks are any better than any other HDD, but I too would consider a swap to an SSD, retaining the original HDD for restore purposes.
From memory I think there was some chat on disc size limits due to the MC200 being fan-less. I'm not a tech wizard so would be interested to know if a larger SSD would produce less heat than a spinning HDD in the enclosure.
1TB will keep me going for quite a while longer, but given the MC200 is now discontinued, backups seem an essential step to ensure longevity. Just not sure how much more "robust" an SSD is....