How to tell iTunes where ALL the songs of a playlists are in the hard disk?

Thanks, yet it did not work. After doing it now all songs show the exclamation mark on the left. As said, clicking ony-by-one I can tell iTunes where they are and then they play, but they are too many to do it that way.

I have the songs in an external drive, which is my booting drive as well. I do not have a single folder will all songs inside, but a folder with sub-folders inside, so the music is sorted that way.

What I would like to do is to keep the standard iTunes music folder on its default location at /Users/~/Music (which in fact contains some songs). But at the same time keep my main music collection at other place as indicated above and be able to play it with iTunes. I do not know if that is possible, but should be since I could play that way before changing the booting hard disk.

Any hint most welcome.

Update:

Even worse. Even if I select the folder where is one of the long playlists (the songs are inside) and set it as "iTunes/Preferences/Advanced/General/iTunes music folder location", iTunes seems to "update" the playlist, yet the very same problem persists: the songs show with the exclamation mark on the left and I must locate them manually one-by-one for iTunes to play them.

Curiously, the songs which I locate manually are always available and play. But how to do it for all songs on a particular folder and for all folders indeed of my full music collection?

I know that I can drag and drop the folder containing the long playlist over iTunes and create a new playlist to avoid such problems, but as said, then I would lose my ratings, sort by last played, etc.

Any idea? Thanks.

Update:

If I double click a song inside the iTunes playlist of the Mac and follow the promt to tell iTunes where the song is (songs with exclamation marks that do not play double clicking them), then iTunes remembers where it is from that moment and plays it OK, even if I change the location of the folder containing the 1,000 songs.

Even more: if I change the location of such folder (for instance, drag and drop it on the Desktop), iTunes updates its location automatically on the fly. But then, again, iTunes only plays the songs that I have manually localized after double clicking on them, but all the other ones remain with the exclamation mark ⚠.

Now I have NOT selected both:

Keep iTunes music folder organized
Copy files to iTunes music folder when adding to library

Is there a way to tell iTunes where all the songs are at once without having to do it manually one-by-one? As said, selecting the folder containing such songs in "iTunes/Preferences/Advanced/iTunes music folder location" does not seem to work becase the exclamation marks remain and double clicking a song inside the iTunes playlist window does not play it but prompts to localize it! They are 1,000 songs inside such playlist folder...