Thank you Dianne and Virtual1. Your replies are a classic demonstration of the great value of this forum! I use the Mac Pro for a lot of reasons and started using the pocket CDs as a handy transport medium, but I just didn't feel right about putting them into the iMac's CD/DVD slot. You've authoritatively confirmed that my intuition was correct. BTW, I've gone to flash drives as a replacement. The pocket CDs are rarely used anymore, but I have a startup disk for backup software that I want to use to restore documents to a new Windows 7 installation on VMware Fusion 3 on the iMac. I think I'll use the Mac Pro to burn it to a standard CD and see if that works on the iMac. If there's a trap in that method that I'm not aware of, please let me know.
Mac Pro dual Quad-Core Intel Xeons Early 2008; 16GB RAM; MacOS X 10.11.6, iOS 9.3.5