A CD file will always show as "Read Only" since you can't write to that medium.

It's odd that you can't open it though. Unless your CAD needs to write a message to the file as it opens perhaps. Try copying the file back from the CD into your hard drive and then open the copy. I had some software that would not run from the CD I'd backed it up onto but it would run fine once copied to the hard drive and started from there.