Write an article based on this summary:

Insert the USB drive into a USB port on your Mac. Check the amount of free space on the USB drive. Open iTunes or the Finder. Select the files in your iTunes library that you want to transfer. Drag the selected files to the USB icon on your desktop. Wait for the files to finish transferring. Drag the USB drive to the Trash. Remove the USB drive from the Mac.
Avoid using USB hubs, as these may slow the process down significantly. You should see your USB drive appear on the desktop.  If your USB drive does not appear, see the Troubleshooting section below. A USB hub is an external device that allows you to plug multiple USB devices into a single USB port. It usually says how much free space is available below the USB drive on your desktop.  Most MP3s are about 3-5 MB in size or 1 MB per minute of audio.  Though this may vary depending on the quality of the file.  See the chart at the bottom of this article for a breakdown of how many songs can fit on a USB drive on average. Alternatively, you can right-click the USB drive below "Devices" in the Finder and click Get Info to see how much free space is available on the drive. If you're using iTunes for your music, you can quickly copy your music to a USB drive using the iTunes program. If you're not using iTunes, open the folder you keep your music in the Finder. You can transfer any of your songs or albums, but you won't be able to transfer playlists. You can hold ⌘ Command to select multiple files, or ⇧ Shift to select blocks of files. If you're using Finder, open the folder containing the music you want to transfer, then select the files and folders that you want to copy.  This is most likely the "Music" folder in the Finder. The files will begin transferring to the USB drive. The original files will not be removed.  If you're moving files from Finder, hold ⌥ Option while dragging them to the USB drive. This will leave the original file on your computer and create a copy on the USB drive. This isn't necessary if you're using iTunes. Alternatively, you can find the USB drive under "Devices" in the sidebar to the left in the Finder.  You can drag and drop files there as well. This may take a while if you are copying a lot of music files.  The amount of time it takes to transfer music depends on how much music you are transferring, and the speed of your computer and flash drive. After the transfer is complete, drag the USB drive to the Trash to safely unmount it. Alternatively, you can click the Eject icon next to the drive name in the Finder. After dragging the USB drive to the Trash, you can pull the drive out without worrying about corrupt data or damaging your Flash drive.