What area of custom action to trigger an uninstall via InstallScript

I have a installation that I'm not sure why prompts for Disk 1. Knowing that the customer might not have Disk 1 I have to ship along the previous MSI and trigger it using install script using

LaunchAppAndWait("MsiExec.exe", sCmdLine + sValue, WAIT);

Where sCmdLine is /x and sValue is the path to my previous MSI.

I can trigger the uninstall but when I do it states there is another installati开发者_高级运维on in process. Does anyone know what phase of the custom actions I should place my function to remove it properly

MSI uninstall's don't execute the UI sequence, only the Execute sequence. It's not possible to do an uninstall from InstallScript because it's already inside an existing MSI execution context and there is a system-wide mutex that allows only one at a time.

I need to understand your exact scenario a little better to give a more useful answer.