That big, if it's not already in good shape, organizing the collection is likely to be an overwhelming task.
Rather than dilute the library you already have into insignificance, create a NEW library by holding down the option key when you start up iTunes and following directions. Be sure you have enough space on your drive, and that "Keep iTunes Media folder organized" and "Copy files to Media folder when adding to library" are checked in iTunes advanced prefs.
I'd do the operation in chunks.
Start with 10 gigabytes of music and see how long that takes.
Work up from there, cautiously. A failed import can leave a mess that has to cleaned up manually.
Moving a terabyte of sound files into iTunes can
easily take as long as a complete backup of a TB USB hard drive.
Before starting, ask yourself whether you really
want that much music. What percent of what's on your USB is actually worth listening to?
See:
Sturgeon's law.