How can I convert a DMG to ISO on Mac OS X (preferably for free)? I have medical records within Windows software within a DMG disk image. How can I convert the DMG to an ISO format on Mac OS X? Is it possible to do it within Disk Utility or CLI easily, or do I need an external utility?
You can use hdiutil to convert from .dmg to .iso directly:

$ hdiutil makehybrid -iso -joliet -o [filename].iso [filename].dmg


Renaming .cdr to .iso is bad advice! An .iso has a slightly different format that allows PC users to play the DVD as well. A renamed file will work on macOS, but NOT on a PC.

If you have a .cdr and want to convert it to an .iso, execute:

$ hdiutil makehybrid -iso -joliet -o [filename].iso [filename].cdr