if your disk space is low, then you are just going to have to release space on it - such as deleting old or unwanted files, deleting your temporary files (or cache) in your browser and to empty your recycle bin. You can also delete the tester fxt files generated for previous runs, in order to make space for new files.
You can check this if you press in metatrader (mt4) fileopen file folder, and then in mql4 folder, you press properties, then go to security advanced and see write permissions.
Mql4files ( for a live expertadvisor or custom indicator or script mode of operations ) or mql4testerfiles ( in case of using expertadvisor or custom indicatorinside a back-testing framework of mt4 strategy tester).
The function is intended for writing of data into a csv file, delimiter being inserted automatically unless it is equal to 0. After writing into the file, the line end character rn will be added.
i had that problem yesterday, when updated win10 and metatrader 4 920 the solution was to restore previous version of metaeditor.
fileread and write just access files in the own mt4 installation sub folders.