File cannot be opened for writing

I have a bunch of files that when I try to edit them, I get the error message "File...cannot be opened for writing." The file extension is m4a, but Verizon Rhapsody music player says they are AAC files. I don't really know what the difference is, but the FAQ says that this mp3tag should be able to edit AAC files.

Is the problem that the file type isn't supported, or could there be something else going on?

If your file is opened anywhere else, it will say this. So make sure the file you are trying to edit is not opened on other software.