ITunes Media And Removable Storage Issue - Can't Get iTunes to recognize the new location. Any tips?

Question

I've had my iTunes media on an external HDD for years.  Let's call this drive B.
My iTunes library and xml file reside on my Mac HDD.
I decided that I wanted to buy a bigger HDD and move all of the media to it.  Last night I copied all of the media files to the new HDD (let's call it P) from B.  I also copied the iTunes folder from my Mac HDD to P and directed iTunes to look for the media in the Advanced settings.
Of course when I launched iTunes after all the hours it took to do all of this copying, it could not find any of the media on P without asking whether I wanted to locate it - and the library was full of explanation points.  When I examined the info on a few songs, it was showing that all of the media was still pointing to B as the location with the complete directory structure pointing to that old location - and not to the new drive.
I tried creating a new library and still could not get the library to recognize that all of the media was now on P instead of B.
I remember when I did something similar years ago, I just changed the location in iTunes in the Advanced settings and iTunes realized that all of the music was on B instead of its prior location.
I finally gave up and restored things back to the original settings with all the media on B.
I did a lot of searches and followed everything explicitly -- which basically meant copying the iTunes folder (with the library and xml files within it), and the media to the new drive and then telling iTunes where the new location was.  To me it shouldn't be any simpler than that to get iTunes to recognize a new location for all the music.
Can anyone offer a more rudimentary breakdown of things for me to try so that I can get iTunes to acknowledge that all the media is on P and not B?
P is larger than B  for those of you who are wondering why I'm trying to get this to work. And P also will be a drive that dedicated to iTunes and media and nothing else.  Unlike B, which has all kinds of assorted files on it along with my music, etc.
And yes, each time I made changes  - I restarted iTunes

Answer 1

Thank you both for the information. 
It sounds like I did something out of sequence based upon reading the split library information, thinking I was doing the right thing.
This is exactly what I did
I copied the folder with all the media from B to P
I copied the entire iTunes folder from the Mac HDD to P
I option click started iTunes and connected to the library file and when iTunes opened it was OK - and looked like it normally does when I was still using B as the storage
I connected to P
When it didn't work, I tried to create a new library and did the same exact things.
So let me ask this:
Should I hook up P again and delete everything off of it except for the folders that have the media in them and then copy the current iTunes folder over onto P again - and then copy all of the media from it's current location into the iTunes Media folder inside of the iTunes folder I copy over from the Mac and then try to connect and consolidate everything?
I assume that if I move everything from the current media folder into the iTunes/iTunes Media/ that I copy over everything will be under the iTunes umbrella as if things were never split?
I don't seem to recall it being this difficult when I originally set up things on B, as it was a split situation then too, if I'm not mistaken.
I really appreciate the instruction.  And honestly it does make sense to have everything in one location to prevent problems like this from happening  - it would just be great if iTunes had a feature that moved and/or copied the necessary files to the new location to prevent splitting links