Okay, that title was a trick to get you to read this post.
The fact is you can’t get the value of SRCDISK in an InstallScript custom action in a Basic MSI project. At least I’ve never found a way to do it. The need arises when you have to access files on a CD or DVD during the installation. So, what do you do?
If you created a release that uses Setup.exe, you can access SETUPEXEDIR in the custom action. It will contain the path to Setup.exe.
That’s great you say, but I’m not using Setup.exe. I have a Single MSI release. In that case, you will have to write a custom action that enumerates the drives on the system, and look for a unique file. A file that will be present on your CD, but not on the user’s system.