Last night, I saved my test level!
Unfortunately, it uncovered further complications.

-=-=-

I thought I could easily use the following method.

1. Open level file
2. Jump to position of level
3. Write level
4. Close level file

And that worked.. .. On Windows!
And then I double checked it on 3DS, only to find that when I wrote the data, then closed the file, the file would "end" at the position of the last write.

With that quirk in mind, I'm going to have to juggle a few bits and pieces of data, and add extra checks, to ensure that the data being saved to the file is correct.
A couple of "total levels in this file" values at the top and bottom of the file should suffice.

*ugh*
This isn't an easy task!!