I would like to add to this discussion with a related point but not exactly the same thing, but it's important to know about it. Sometimes Terragen will appear to be finishing a render, or it might have already finished and POOF it goes waay (crash). When this happens to me it's not a memory issue because I have 128GB and never see my memory going much over 64GB of use.
What I suspect is because I don't always pause the RTP it will start back up after my renders finish, and if RTP runs into a bug somewhere while refining it could cause a crash. I suspect this because I do get my render frames saved, but when I come back to my PC and check Terragen is not running. Of course I get scared, and check to see if my frames were saved and they were. So even if Terragen crashes, always check to see if your frames were saved.
There is also an option in Preferences (File Saving) to save all renders even if you don't chose "Render All to Disk" or "Render sequence" from the Render Panel.
-Derek