If you go to Start -> Control Panel -> Administrative Tools -> Disk Management, you should be able to right-click the drive and assign the letter H: to it, assuming of course that the letter H: isn't already assigned to something else. Depending on what software you're going to use with the drives, you might need to label all 4 the same if you plan to interchange them.
I have 4 laptops running windows 7 and 4 external HDDs. I need to be able to use any HDD with any laptop but for them to always keep the same drive letter (H), and yes.. I have to use the letter H. Can this be done with an autorun.inf? or is there a more simple way that I have overlooked?
Side note, I can not use any 3rd party software to achieve this.