Thanks for the fast response!
Let's see what I can achieve now that I have a concrete example.
1) Created bespoke column called "cfolder". This is essentially "Category", only I didn't use that word because I thought that it's probably already in use.
2) Assigned values to cfolder for the books in library, based on the names of the folders on my memory card. I.e cfolder="Fiction"
3) Looked at the Plugins, after reading dwanthny's advice. How can I find the plugin that you are talking about?
4) Looking for "Send to Device Card A" that TheDucks is talking about. I remember seeing it in Calibre 7, but now I am on v8 and I can't find it... Continuing to look...
5) Modified the string for the template, but I am not sure if I did it correctly. How is it going to know what drive the folder is at? I can't figure out how to set that.
6) In the Calibre Library, I am right clicking on the name of a book, and selecting "Save to Device" "Main Memory"
(Memory card A and B are greyed out (?) even though there is a memory card in the Reader)
This creates a new folder called "Books" which I did not ask for. In this folder, Calibre creates the folders that corresponds to the value that I entered in "cfolder".
This is almost exactly what I wanted to achieve, although I'd rather not have the "Books" folder, but save straight into the folders that I specified. Where is it telling it to use this "Books" folder? Or maybe that is something specific for my Reader?
Secondly I don't understand whether Windows / Calibre recognises my Irex reader and it's 16 GB memory card, or not.... Or does it think it's one and the same?
When I connect the Irex Windows displays the Irex like a 16GB drive. Calibre knows it's an Irex, but it doesn't seem to realise that the reader has it's own memory, and then there is an SD card. How can I "educate" Calibre about this, are there any settings etc?