ITUNES - 3 QUESTIONS: PLEASE HELP!

Question

1 Before deciding to buy an EHD and copying my itunes library over (as back up) my music files fitted on my PC hardrive (C:) with no problem.  However it would seem that since the transfer some random files (tracks) are not available without the EHD connected to the PC.  The strange thing is that there seems to be be no pattern or logic to why these tracks are no longer sourced from my C:  They are just odd track(s) within an album (rather than entire album).  But there are many of them.  The 'skeleton' file (or track) remains listed in the library but when you click on it to play an exclamation mark icon apperars next to the songtitle.  When you then plug the EHD in, it plays as if it did before I got the EHD.  Why is my PC now dependent upon the EHD to find and play only some tracks?
2. I have connected the EHD to a new PC and copied the entire library to a new downloaded and thefrore empty itunes.  All tracks appear in the new library and play fine,  but they are not organised into their original themed playlists.  The playlist section (to the right) is just empty.  These playlists are very important to me and I have spent many hours/days organising them from 10,000+ tracks.  Is it possible therefore to copy these also to and from the EHD so they can re-appear and get backed up to a new PC.  My biggest fear is not just loosing the library but the playlists in which the library tracks are organised into.
3.  I spend much time editing my library after I have imported discs.  Sometimes I will import many CD albums but delete unwanted tracks over time (as and when I get the time to listen to them).  However although they remain deleted from the library on the C: they do not automatically delete on the EHD (p:).  Is it possible to 'sync' the two so that all importing and subsequent editing activity automatically updated when the EHD is connected.  Similarly to how the ipod syncs with itunes when connected to the PC.  (i.e the ipod recognises automatically that some tracks have been deleted since last connected and they are therfore deleted of the ipod library).  It just mirrors whatever is on the C: when connected and therefore adapts to all import/playlist sorting/track deletions activity since when it was last connected.
I hope my explanations and related questions make sense as I am not very IT literate!! 

Answer 1

The iTunes software like many different music applications tend to organise music into a folder structure which is usually like this:
MUSIC FOLDER -> ARTIST -> ALBUM TITLE
if it is a compilation album the ARTIST is usually superceeded by ALBUM ARTIST often called either VARIOUS ARTISTS or COMPILATIONS .. (you can use either in iTunes and depends on whether or not you choose to tick the compilation checkbox)
Anyhow putting all that to one side for a moment it is usually best to leave the structure of the songs as they are on the drive but then create different VIEWS of them by using either playlists or smart playlists.
My advice would be to leave iTunes organise your music and then create the views in the iTunes interface.
If you use iCloud and iTunes music match the good news is that you can have upto five computers and unlimited iPad/iPhone and iPod devices pointing at all your music and if you create a playlist view on any device then the same playlist automatically appears on every device.. Note this is true only if your music is stored in the cloud. (which costs about £22 per year for 25000 songs (not including any you purchase from the iTunes store))...
However, if your music is stored locally only, on say a PC, you will have to 'sync' that playlist to your devices usually by cable (though can be done over wifi) and you will have to recreate the playlist on again on your other PC.
I am not sure if that answers your 2nd question?
As regard to your third question you should not have to edit your songs albums after you import them into iTunes ... I would advise you do this before importing ... Take a look at the following three items of software:
MP3Tag
Albumart Downloader
Beatunes