So I have been trying to play Fallout New Vegas from my secondary hard drive. When I upgraded my hard disk I left the old one with all the data still in it and am using it as a secondary. When I try to run the game it tells me to install even though all the files are there on the E: drive. Is there an option I have so that the games looks for the files on the E: drive instead of the C:?
From the comments, the game was originally installed on the primary Windows hard drive, but then was relegated to secondary status. In those cases, even though the game was installed, the extraneous configuration changes the installer made are no longer accessible to the new installation of Windows, and it doesn't know what to do with them.
Your best bet in these cases is to reinstall (or, if the installer allows, repair) the game, over top of the old files. That will refresh the configuration changes, and allow you to properly play your game again.