Ok, you are using A6 wdl and I use A7.80 lite-c...
As far as I can remember you should delete the str_cpy(save_dir,...) line and define the save folder as you've had before. You could try instead of savedir "\\sd"; the following:
savedir "sd";
SAVEDIR "sd"; (was/is A6 case-sensitive? I don't think so)
string savedir = "sd";
and
file_open_write("%SAVE_DIR%\\fi.txt"); (this may only work in A7).