We're currently in the middle of an install blitz at Microsoft.  The VS Setup team is trying to get 1500 internal folks to install a recent build.  We're aiming to validate our Setup works correctly and at the same time find any potential show stoppers.

During the blitz, we've seen two people run into an Express installation issue.  They are trying to install Express on a different hard drive that's not their main OS drive. 

The main OS drive doesn't have a lot of disk space remaining so they would like to install the entire product on a separate drive.  However, when installing the Express Products, you can install the Express product itself on a different drive but the Framework needs to be installed in the Windows directory of your system drive.  There are also some shared components that also need to be installed on the system drive.

Some folks have reported this as a bug but it's by design for the Express SKUs.  Make sure you leave enough space on your System drive when installing the Express products.