Thanks for the help!
Here are the results of running GSmartControl. No errors, but test aborted... I will run it again, just in case it goes through this time, and will report if it does.
Nothing highlighted in Attributes (but entries showing 'old age' or 'pre-failure').
Something I noticed by looking at the CBS.log, which might or might not be relevant (I am not technical enough to say):
It looks like the system now does find the install.wim file as I am not getting the 'The alternate source path directory could not be accessed' messages any more. But it looks like the system cannot find the specific files it's looking for.
As an example, taking the first file in the log, shtransform.dll: DISM seems to be looking into ‘x86_microsoft-windows-s..formers-shell-extra_31bf3856ad364e35_10.0.18362.447_none_e3ca34a956641477’. But that folder does not seem to be in the install.wim that I am using to run DISM, though, so no surprise DISM cannot find it (I mounted install.wim so can see the files inside). A file with the same name is in there, but in a different folder.
The folder does exist in my original WinSxS directory, but empty.
I tried a couple of other random entries, and same result: the file exists in install.wim but not the folder listed in the log (i.e. the file is in other folders…); that folder exists in my original installation (or in some cases the folder name only appears as a manifest file), but the file is not in it (it can be found but in other folders).
Does that shed any light on the issue?
Thanks
Laurent