This morning when I connected my iPod to add a few things,  it asked if I wanted to update iTunes,  so I said yes.  This Version 7.3 seems to have this problem:

“itunes library file cannot be saved” an unknown error occurred (-50)

I looked this up online,  and it seems that the iPhone requires version 7.3,  so I suspect that in their rush to get this version up,  once it worked OK with the iPhone,  they released it,  but all the others (Mac Users included) seem to be getting this error.  ON the first couple of forums ,  there were no answers.  Not good.