Dell Bios Splash Screen

The "Dell BIOS splash" screen is the screen you see at boot up when the Dell logo first appears.  As the logo is being displayed the system's BIOS is being loaded. 

 

As for the hard drive diagnostics, I am assuming that you are referring to the quick 90/90 hard drive test.  If so, when the Dell logo first appears at boot up, press the CTRL, ALT and D keys at the same time.  The 90/90 test will then run on the Primary hard drive and give it a Pass or Fail grade.  Click here  for a Dell Knowledge Base article on the 90/90 hard drive utility.  If you are not seeing the Dell logo then simply do the key press right after the system starts to power up.