The iTunes Media folder contains music purchased on iTunes, music imported from CDs and other iTunes files including iOS apps. You can transfer this folder to a USB drive just like you would any other file on Windows 7 or 8 -- drag the folder's contents onto the USB drive. To find the folder's location and make sure that it contains all of your music, look in your iTunes preferences.
Open the Edit menu in iTunes 12 and choose Preferences. If you can't see the menu bar, press Ctrl-B to show it.
Switch to the Advanced tab and copy the iTunes Media Folder Location by selecting it and pressing Ctrl-C.
On the same tab, note whether the Copy Files to iTunes Media Folder When Adding to Library option is turned on. If it is, you can skip the next step. If not, you don't need to enable it but follow the next step to consolidate your audio files.
Open the File menu, mouse over Library and choose Organize Library. Turn on Consolidate Files and click OK to copy music files scattered around your computer into the iTunes Media folder.
In Windows, open the iTunes Media folder using the location you copied earlier. The fastest method is to press Windows-R and paste the location into the Run window by pressing Ctrl-V.
Afterward, press Windows-E to open the Computer window, which shows your connected USB drives. If you haven't already done so, plug in your flash drive now.
Select the media folders you want to copy from iTunes, such as Music and Tones, and drag them onto your USB drive's entry in the Computer window.
To copy your entire library, select and drag everything from the iTunes Media folder.
Music and apps can take a lot of space. If you have a large collection of either, your library might not fit onto a flash drive, prompting an error when you try to copy it. Open a subfolder within the iTunes Media folder, such as "Music," to copy individual items if you can't move the entire library at once.
You can re-download songs purchased on the iTunes Store after moving to a new computer, even if you don't transfer your old library with a USB drive. Log in to iTunes on your new computer, open the iTunes Store and click "Purchased." Click the cloud icon to download a song -- you won't be charged again.
Using the Consolidate Files option creates a new copy of each song in your iTunes library that wasn't already stored in the iTunes media folder, a process that can use a lot of hard drive space. After you finish your transfer, you can safely reclaim that space by deleting any music files outside of the iTunes Media folder that also appear in your iTunes library.
ITunes includes a menu option titled "Export Library," but if you're trying to copy or move your music to another computer, this option isn't what you're looking for. "Export Library" only saves a list of your music and metadata -- not the audio itself.
You can listen to music stored on your USB drive, but if you're moving to a new computer, you should copy the files onto the new computer's hard drive. Playlists made using files on a USB drive won't work after you unplug the drive.