If you have not done so already, you can download TouchCopy from here:Download TouchCopy Now
If you have different iTunes libraries on two computers, or external hard drives and you wish to have all your iTunes music in one place, TouchCopy can help you...
First, use iTunes to load up your iPod, iPhone or iPad with music from your first computer. If your music is contained on an external hard drive, you will need to import it into iTunes by selecting the "File" menu in iTunes, then "Add Folder to Library" and choose the location of your external hard drive.
Once your iPod or iOS device contains all the music from your first computer or external hard drive, use TouchCopy to copy music from the device into iTunes on your second computer, which effectively merges your iTunes libraries.
Finally, use iTunes on your second computer to re-sync your device, which will put your merged music library onto your iPod or iOS device.
If you would like to have your merged music library on your first computer as well, simply use TouchCopy on your first computer to copy your device's music to it.
Purchasing the lifetime version of TouchCopy allows you to install the software on two different computers. TouchCopy can be used with an unlimited number of iPods, iPhones and iPads.