Firstly, cool scenario. From your description I was expecting (dreading) something over 12,000AU and expected that to be the issue. In comparison the 6,000 AU scenario you're working on is well within the bounds of what is possible for most systems to run (although it's never going to be a speed demon)
I've saved it a few times at different points without issue; something I noticed in your first post is mention of 'both steam and non steam'; is it possible that you are saving in the wrong folder? Having two installs of CMANO has caused issues in the past if I remember correctly.
The next question is what exactly is happening when a save 'fails'? With a scenario of this size the save process will take a couple of seconds to complete, are you getting that 'pause' each time with the failed saves or is something else happening?
It's still a work in progress, and it's really for a future new-build machine.
Yes, I'm saving in the non-Steam scenarios folder, and as a back-up, copying it into the Steam scenarios folder.
I do this back-up through file explorer and not the simulator.
I have that pause each time, fail or not.
What happens is I save it, then continue to build. At some point, the save fails. This is happening even after, say, 5-10 minutes of building then saving.
I'm conscious of the size of the scenario and my system memory limitations, which is why I'm only building for a short time, then saving, then exiting, and re-starting.
Should I delete the Steam version?