I backed up my PC's iTunes folder to an external drive after selecting to consolidate files, etc.
I connected the external drive to my Mac and did a copy and paste of the iTunes folder. I have not put any music into the iTunes on the Mac, so I just said Yes to overrite (it said there are 2 files that will be overwritten- I think they are apps I downloaded this weekenad i am fine to overwrite them). It starts to copy and then I get the error message: "The operation can’t be completed because the item “iTunes Library Extras.itdb” is in use." I verified that I do not have iTunes open on the Mac and made sure that when I backed up from PC, I did not have iTunes open there either.
There was an iTunes folder on my new Mac. I may have installed it. I just deleted the whole iTunes folder under Music. I then tried copying and pasting from my External Drive again and now it is copying the data as I would expect.