Where the MSI file is copied after the installation?

Does it not go into %windir%\installer\

Though I think that the files may get renamed. Not sure where you get the name mapping from...

This directory gets very big so I move it to an external drive. This sometimes cause uninstalls or updates to fail with a missing msi error, but this can be fixed by putting the directory back

Installshield creates a copy of *.msi files at %windir%\installer\ path where %windir% is an environment variable which usually points to C:\Windows\. Before creating a copy of *.msi files it renames it using some random nomenclature e.g. I can see a file named 65ec5c99.msi in my C:\Windows\Installer directory.

Now to figure out the actual product name for the renamed MSI file there are two ways:

1. Check LocalPackage attribute inside registry at path - [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersio‌​n\Installer\UserData‌​\<InternalUserId>\Pr‌​oducts\<ProductGUID>‌​\InstallProperties]. InternalUserId is something of sort S-1-5-18. ProductGUID is a GUID like 00058CD18F0BF523DA1072073D56715D embedded as a public property inside the MSI file itself. You can check this public property by opening your MSI file using any tool e.g. orca

2. Open C:\Windows\Installer folder in explorer. Change the view of the directory to Details view. Add Subject column to the view. Now the product name can be seen in Subject column as shown in the screenshot below:

Note: C:\Windows\installer\ directory is not visible in explorer until we check Show hidden files, folders, and drives and uncheck Hide protected operating system files(Recommended) advanced settings under view folder options as shown in the screenshot below:

You can force a recache/reinstall using with MSIEXEC, the recommended way to update buggy installation packages that cannot be otherwise removed is to recache with a fixed package, then uninstall as usual.

MSIEXEC /fv setup.msi