How do I transfer old iTunes library with ripped music from Windows PC to Mac without -50 error?

Before getting my new Mac I transferred my old iTunes Media library, on my Windows PC, to an external storage device. This library contained both purchased music as well as music ripped from CDs. I cannot seem to be able to transfer this library to my Mac. I always get a -50 error code, but I cannot find out what the problem is. My purchased music has been transferred from the cloud, but I’m keen to get access to my ripped music. Can anyone help?

[Re-Titled by Moderator]