Error while wrting data into file using Applescript

Question

am using the below script for writing the file, but sometimes am getting errors like mentioned below, pls suggest..
set filepath to POSIX path of "Macintosh HD:Library:Application Support:Macromedia:mms.cfg"
try
    tell application "System Events"
        if file filepath exists then
            set myFile to open for access file filepath with write permission
            set fileData to read myFile
            set eof myFile to 0
            write "blah blah" to myFile
            close access myFile
        else
            return "File Not Found"
        end if
    end tell
on error
    return false
end try
Error:
"Network file permission error." number -5000 from file "Macintosh HD:Library:Application Support:Macromedia:mms.cfg"
Also some times i will get this error, am unable to close the opened file
"File file Macintosh HD:Library:Application Support:Macromedia:mms.cfg is already open." number -49 from file "Macintosh HD:Library:Application Support:Macromedia:mms.cfg"
When i tried to close the ofile am getting this eror:
on openAFile(filepath)
    try
        set fp to open for access filepath with write permission
    on error errstr number errNum
        if errNum = -49 then
            close access filepath
            set fp to open for access filepath with write permission
        else
            display dialog errstr
            return false
        end if
    end try
    return fp
end openAFile
set pointer to openAFile("Macintosh HD:Library:Application Support:Macromedia:mms.cfg"
set fileContents to read pointer
Error
"Can’t make \"Macintosh HD:Library:Application Support:Macromedia:mms.cfg\" into type file." number -1700 from "Macintosh HD:Library:Application Support:Macromedia:mms.cfg" to file

Answer 1

if you're using HFS (colon-delimited) file paths, then you always have to prefix it with the file or alias keywords.  If you don't, Applescript will see it as a plain sting rather than a file path.  You can use POSIX (slash-delimited) paths directly.  examples:
set filepath to POSIX path of file "Macintosh HD:Library:Application Support:Macromedia:mms.cfg"
set fp to open for access alias filePath with write permission