The CD-ROM standards prohibit the use of punctuation (such as periods or hyphens) or lower-case letters as either directory names or filenames. The only characters allowed for CD-ROM are A-Z, 0-9, and the underscore ( _ ). For example, the filenames FILEONE.TXT and FILE_ONE.TXT are OK, but FILE-ONE.TXT and FILE-1.TXT won't work.
Also, when laying out the file and directory structure of your title on the disc, make sure to limit the number of files in a directory. If you have a large number of files in a directory, this can significantly increase the time necessary to open a file.