I am posting this solved issue in case it helps somebody.
Cube has many "Import" options, plus "Open file" options. However, while this should be absolutely totally bloody trivial, none of them worked. It would complain about Can't overwrite an existing project, or a button was greyed out without an explanation. Well, it was written in Java, and Java programmers are paid $10 per opcode
I have Cube v1.6.1 (the latest, I am not updating anymore) installed on two machines.
The existing project is on one, under c:\projectname\project1.
In it are these files and folders
12/05/2021 18:53 169,656 .cproject
05/10/2020 15:37 84,465 .cproject_org
07/12/2020 19:00 1,744 .project
05/10/2020 15:37 1,898 .project_org
09/04/2021 20:51 <DIR> .settings
09/04/2021 20:51 <DIR> CMSIS
12/05/2021 18:53 <DIR> Debug
09/04/2021 20:52 <DIR> HAL_Driver
10/05/2021 16:24 <DIR> inc
14/09/2020 15:55 447 Kxx-SWD.xml
28/10/2019 12:41 440 Kxx.xml
12/05/2021 18:58 6,673 LinkerScript.ld
09/04/2021 20:52 <DIR> Middlewares
10/05/2021 16:24 <DIR> src
09/04/2021 20:52 <DIR> startup
07/05/2021 11:39 8,916 TestCPPProject (1).launch
and that seems to be standard.
On the other machine I created c:\projectname\project1 and copied the above data there.
Of course I googled all over the place, like everybody does, and this is a common issue, all over the place, but all the advice is old, from say 2017, when it was different software. Just different enough...
Then I had an idea. Even when I copied the data into project2, hoping to import that into a project in project1 (project 1 dir did not exist at the time!) it complained that project1 already exists, so obviously there was some reference to "project1" in the project files... So, maybe Cube is storing
project config somewhere else
also, not within the above project directory (folder). Perhaps in the registry, which would be a bit messy to copy over. But it was simple. When I copied over the following rather usefully named directory (btw the original install was 1.4.0, then updated online to 1.6.1)
C:\ST\STM32CubeIDE_1.4.0\STM32CubeIDE\configuration
from the source machine to the target one (obviously with Cube
not running) and started up Cube on the target one, it just came up, whole project on-screen
The obvious implication, unless I am doing something wrong, is that to backup a Cube project you have to back up not just the project directory but also the above Configuration directory in c:\st.
You might still have fun with the debugger not connecting, but again that one (esp. the cryptic "joined drives" error) is aaaallll oooover the internet too
On the win10 machine it went away after some reboots, and ticking all the boxes for private and public networks for the debugger, when the win10 "do you want to give this thing access" window pops up.
Win10 installs also don't show up the weird bug I get on win7-64, where you see a load of blank lines when the project is being built. Looks like a display driver issue.