that's because you've got a tutorial that must be well over a year old and at that time there was no "bak" file - it did not exist then - and only appeared in a later system

As far as extra space is concerned remember that midi files do not use the Technics folder system, so even if you have 99 folders full and space left on the card this can be used for midi files. The midi playlist and playback system is very comprehensive compared to what you have been used to off floppy disk up till now.

And that extra space is definitely needed for audio!