quite some time ago this user was told to turn off their "copy into library when importing" option, due to a small internal hdd
And that is indeed the
correct advice when storing files
outside the default "home/music/itunes/itunes music" location... else we would wind up with needless (wasteful) duplication. I've had that option turned off ever since iTunes version 2, when i moved all my music to a dedicated partition.
Downloads from iTMS and rips from our CDs will get stored in the (relocated) media folder as expected... so it's only when adding items that are
already sitting around locally somewhere that we would need to physically move items (if so desired).
For someone with such a humongous library, sounds like this person never put much thought into what was actually going on behind the scenes with the song files. Even though iTunes and Mac OS "just work"... some initial organization is required when wandering outside the default setup [e.g., having the media folder be something other than "home/music/itunes/itunes music/".]
While iTunes was initially intended to handle such a "scattered files" scenario as you described, it behooves the user to keep stuff in as few locations as practical (if for no other reason than to facilitate regular backups and potential restore operations).
Now that we have all the files in one place, itunes needs to get the message where all the files went. It doesn't even do you the courtesy of telling you it lost a file until you try to play it.
Well, that's true...
however, there is a very simple way to force iTunes to study every single filepath from scratch: just do an "
Add to Library…" and then select the 'media' folder itself. iTunes will scan every file in the hierarchy [whichever folder we choose] and fix whatever paths it can resolve (if they need fixing), or put exclamation marks on the ones it can't find. I highly recommend doing this only immediately after launching iTunes. [i.e., don't do this after iTunes has been open for a long time]. I say that because i once saw iTunes trip and make its library file double in size. [that happened a while ago, so maybe that bug has been fixed... but better safe than sorry. So, i only do that "Add to Library…" operation on the entire media folder immediately after a fresh launch.]