I have a post on reorganizing things before attempting to move them from one computer to another which runs as follows:
Make a split library portable
Here are the typical layouts for the iTunes folders:
In the layout above right, with the media folder (everything in the red box) inside the library folder, the library is considered to be portable. A portable library can be moved from one path to another without breaking the links from the library to the media, and being self-contained it is much easier to backup and restore. (You do backup, don't you?).
You can rearrange things to make a split library portable by taking a number of small steps which don't break the library.
Before you start any media files that are outside of the media folder will need to be consolidated. If the library is in the old style layout then it should be upgraded to iTunes Media Organization (Library > Organize Library > Rearrange files in the folder <Media Folder>) to ensure that iPod Games, Mobile Applications etc. are brought inside the media folder, otherwise the links to these won't survive changes in the path of the library.
The basic non-fatal manipulations are:
- You can create or connect to an alternate set of library files by holding down Shift (Win) or Option (Mac) when starting iTunes. (Note iTunes will continue to use this library until you use the same method again.)
- You can move the library files to a new location and connect to it there as long as the media stays put.
- You can move the library files and the media together if the media folder is a direct subfolder of the library folder.
- If you have already moved/copied the media content from a subfolder of the library folder to a different location then you only need to copy the library files for it to appear as if you have moved the entire library in the way allowed above. I.e. just copy the library files into the parent folder of the media folder.
- You can rename the media folder to iTunes Media (if it isn't already) if the media folder is inside the library folder.
- iTunes uses the name of the folder holding the library files as the window title. Having made a library "portable" you may need to take a final step of renaming the library folder to iTunes or, if the library files have ended up at the root of a drive, moving all of the library files and content folders into a new folder called iTunes.
IMPORTANT: After each change you need to open, test and close the relevant library before attempting another change. If a change broke the library, undo it or revert to using the previous set of library files.
In essence all you need to do to join up a split library and make it portable is copy the library files into the parent folder of the media folder on the external/secondary drive and use the hold-down-shift/option-when-starting-iTunes method to connect to it. Other manipulations may be required to normalize the library so that the library and media folders have standard names.
In your case I'm suggesting that you can use 2. to relocate your library files to a faster internal drive while leaving the media on the NAS (splitting the library), and that you would then use 4. to join your split library back together again before attempting to move it to a different drive.
After using 2. you would need to make sure that under iTunes Preferences > Advanced the media folder was pointed to iTunes Media on the NAS. There is also an issue with such split libraries that iTunes can sometimes reset the media folder if the NAS/external appears to be offline when iTUnes starts up.
tt2
