is a string:
Dim HDD_Serial As String
So you don't need to call
on it - it does nothing:
Secondly, when you assign a value to a string inside a loop, you only get the last value set once the loop is completed.
So you only process the last value returned by the MOS. And if that doesn't have a serial number ... what happens then? We can;t tell what is returned - we don't have access to your specific system. So, it's going to be up to you.
Fortunately, you have a tool available to you which will help you find out what is going on: the debugger. If you don't know how to use it then a quick Google for "Visual Studio debugger" should give you the info you need.
Put a breakpoint on the first line in the function, and run your code through the debugger. Then look at your code, and at your data and work out what should happen manually. Then single step each line checking that what you expected to happen is exactly what did. When it isn't, that's when you have a problem, and you can back-track (or run it again and look more closely) to find out why.
Sorry, but we can't do that for you - time for you to learn a new (and very, very useful) skill: debugging!