error on itunes "Invalid Drive: G:/"

I recently purchased an iphone 4s, and when trying to connect to itunes it said i need to update to the latest version (I was on version 10.3, and it was on my external hard drive). When i went to do this, the installer wouldnt work, as it said "Invalid Drive: G:/" can anyone please help me update my itunes without deleting the whole thing so I can finally get music on my iphone! Thankyou!