In your iTunes Preferences, you need to switch from the C:\ Drive (if you're using Windows) to the Drive Letter of your external HDD, This might clear things up for you. For more info, here is how to move iTunes to a external HDD from CNET.
i have all my music on a external drive which has worked well up to a year ago when all wasn t recognized,i hav just spent the best part of five weeks clicking every song so it registers,now tonight it has done it again .Surely there must be a quicker and more straight forward way of fixing this problem PLEASE HELP

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