I changed my CD/DVD drive letter after I installed Windows Server 2012 R2 and a couple other features. Then, I wanted to install the .NET Framework 3.5 feature. It kept complaining out not being able to find the source. I had to set the new source location for it to install correctly.

From Server Manager, you add the role as you normally would until you get to the “Confirm installation selections” window:

Click the “Specify an alertnate source path” link and for the path enter the location with the correct directory of your 2012 R2 installation source. Since I changed my drive letter to the Z drive, mine was as follows.


From here, it installed as it normally would.

Note: I also discovered that if you apply some updates and then need to add the .NET 3.5 Framework feature, you might need to uninstall one or two of the updates. In my case, I had to remove update 2966828. I used the method specified here: http://jim-zimmerman.com/?p=1381.