For divMMC I was actually thinking more thinking of loading a block from disk to Speccy memory, show/edit it and writing it back, like a kind of buffer. This way a huge file could be created and edited. Just an idea :)
That won't work at all. Because if you write something at the beginning of the file, the rest of the file will be shifted by the X bytes you wrote... Or rather, it would be possible, but it wouldn't be the fastest...
