Hi Lewis,

Well what you get when you actually try to open a Mathematica notebook written in Mathematica 10 on the new machine is something like this

It should not look like that. Notebooks created in V5 of Mathematica open fine in V12. You may get prompted to convert the notebook to the current style. Edit your post and attach the notebook so we can reproduce the problem and try to identify why that is happening.

What exactly is your end goal? In a previous question you asked on the same topic I suggested that perhaps converting each card to an HTML document with hyperlinks to other HTML documents (cards) might be a better way to do this rather than converting them to notebooks. Did you consider that?

Converting all of the cards to HTML documents and linking them can be done with a Wolfram Language program.

Hi Lewis,

Here is one way

notebook = CreateNotebook[];
fileName = FileNameJoin[{"/User/myName/Desktop/Hypercard Xref", "myString.nb"}]
NotebookSave[notebook, fileName]

If you want to programmatically add content to the notebook take a look at this.

If the notebooks you want to create have a similar structure you can create a template notebook and populate it programmatically. See this.