When you write a CD (or, when someone else sends you a
CD), it typically is saved as "read only", so you CANNOT edit and save the file back onto the CD, unless the CD is running as a read/write drive, which you probably do not have the CD setup this way. I typically will open the file from the CD and do a "SAVEAS" and then store it on my hard drive. Then, after editing the file, you can copy the file BACK to the CD with your CD write software (Adaptec, or other).
In my years of CAD experience, I do not advocate using either a CD or a floppy disc for opening and closing files, you are MUCH better off copying files to your local hard drive, or server, and copying them back to a CD or floppy when you are done.
Note that if you copy the files from the CD directly to your hard drive, you must change the parameters for the file from "read only"- click on the file on the hard drive and click on the right mouse button, go the bottom of the list and click on "properties", then UNCLICK the "read only" button. Then, you can open the file in ACAD and save it under the same name. Let me know how this works out!