Some of the iTunes Store files were not copied to the iPod because the iPod may only contain content from five iTunes Store accounts.

Are you getting this message? I never have. A reader at the Apple Forums had this issue.  Fortunately the solution is easy. Jim has the solution. “Here’s a script that you might want to use.

This script will examine each track in a selection of tracks or a selected playlist to determine if it was purchased from the iTunes Store and then copies it to a discrete playlist named for the person who purchased it, eg: “Steve Ballmer’s Purchased Music”. Recognizes song, video, and audiobook purchases. Purchaser information is gleaned by examining the track’s file, which, depending on file location and access speed, can take as long a second per track.