How to restore the missing Windows Installer cache files (MSP or MSI) and resolve problems that occur during a SQL Server update

Very important article for DBA who involve in SQL Server update activity. If you unable to apply Hotfix, Patches or Cumulative update of Server pack .

And while updating Hotfix, Patches or Cumulative update of Server pack(S1 to SP4) getting error due to MSP or MSI file missing 

To fix this issue please open below link-

How to restore the missing Windows Installer cache files (MSP or MSI) and resolve problems that occur during a SQL Server update

This article has enough details to know the issue, cause of the issue and solution. But one common reason is to missing these file is sometime windows server team delete these file to create free space in Server.