HD Speed is a tool to measure the performance of a hard disk in terms of read time, access time, copy, write, etc. Typically, if your virtual hard disk is mounted, you will have the possibility to test it, but you'll actually test the physical drive since the writing is performed on the disk file stored. The results should be the same since it's the same drive, even if you have a virtual drive. However, to measure the actual performance, test the speed of the physical drive.