The attachments were posted with my original post in the General Forum and were there each time I revisited so I don't know why you couldn't see them?
My screenshots show at what point in the installation the failure occurred. I had to use Task Manager to exit each of these screens. Most were at "Addon Manager\Simobjects\Misc\FSDT_SafeGate\model.FSDT_Agnis_Wall_left_off"
Space is not an issue on any of my drives.
I have 10 Airports and GSX and I can install any of these on my system on it's own (after clearing down all FSDT). But the second and subsequent installs all fail at the point "Addon Manager\Simobjects\Misc\FSDT_SafeGate\model.FSDT_Agnis_Wall_left_off".
So only the first will populate all requirements of the setup - Add/Remove programs, Prepar3D v4 Add-ons, Addon Manager, P3D v4.1 Library, SODE etc.
Any subsequent installs will fail at "Addon Manager\Simobjects\Misc\FSDT_SafeGate\model.FSDT_Agnis_Wall_left_off". These failed installs will create and populate Addon Manager(though whether fully or not I don't know). But will not be shown in Add/Remove programs, Prepar3D v4 Add-ons, P3D v4.1 Library or do a SODE check.
I am not having any other issues installing other scenery products - Orbx, Flightbeam, Taxi2Gate, Flytampa, 29Palms, LVFR, AP2000.
Maybe you could advise me how to completely clear out FSDT from my system and start again from scratch? I have no way of uninstalling using Add/Remove programs as all the failed installs don't appear there.