Or alternatly, before you render save with a temporary name - render file - or some such. I save before every render everyway, so a simple name change to a stand, entirely temporary file isn't much of a problem. That way, I don't create vast ammounts of extra, unneeded files, just one (possibly one in each folder.) So, my routine is to save as normal as I go along, then pre-render, save, then save as, then render. If the render fails, I've not lost anything. Probably due to years of experience of the vagaries of windows, I always make lots and lots of backup saves of anything of importance anyway.