It sounds like you have been playing the music by "pulling" it from the USB drive (sound played from laptop but source file was on the external drive). I'm an old school Windows Explorer guy so I would connect the USB drive, navigate to the folder in which all the FLAC files are and copy/paste them to the Music folder on the new laptop. At that point I would open Media Center and import (I believe that it defaults to importing from Music etc.) You would then have the FLAC files copied from their current home on the USB drive to your new drive for reliability. I believe that you can also do this from within Media Center use the Tools-Library Tools-Rename, Move, and Copy Files option. Good luck- I'm sure that you will get advice from more knowledgeable users.