This message will arise if the program for some reason was unable to store one of the subset files.
Likely reasons are that the device is full or write protected.
Again likely reasons are that the device is full or write protected, or that there already is a read-only file with the name you specified. The values can only be whole numbers. This message can arise when you try to set options in the options window . The values for the otions must be integers, i.e. whole numbers, and nothing else. Check that you haven't typed e.g. a letter by mistake and try again.