Text Edit - Save RTFD as RTF
Solution 1:
The RTFD format is actually a package format that allows it to have related files attached, like images. If your document has any files attached, you will not be able to save it as a straight RTF.
Try right clicking on your RTFD file and select "Show Package Contents" and you will see the contents: the "core" RTF file is usually named TXT.RTF and the other files also present.
You can actually grab the RTF straight out of the Package without even going through saving it manually via TextEdit.app.