I had that too. Steps were from memory: * Delete collection from iTunes. * Change library location. * Change library location back to original location. *

I had that too. Steps were from memory: * Delete collection from iTunes. * Change library location. * Change library location back to original location. * Wait 30+ minutes while iTunes freezes rescanning you library (I haven't used iTunes on Mac but on Windows it is a horrible experince, seriously haven't they heard of threads?).