I second Kevin's suggestions. Make sure you're doing your benchmarking with SQL Server (and anything else non-standard) turned off.
HD Tach may not be the best tool for the job. According to Wikipedia:
As of September 2011, the latest version of this application (3.0.4.0) is still not fully compatible with Windows Vista or Windows 7. However, HD Tach works in Vista and 7 64-bit by running it in Windows XP SP2 compatibility mode.
And...
On December 5, 2011, citing the lack of time to devote to the project, Simpli Software formally announced on its website that HD Tach has reached end-of-life and is no longer being supported.
Have you looked at SQLIO? It too has it's shortcomings but was specifically designed to do the kind of benchmarking I think you're looking for.