After the disaster that was the roll out "Device Library Plus" with 6.8.1, many users started having USB syncing freezes (me included). Pioneer support was less than useless EVEN AFTER TELLING THEM EXACTLY WHAT THE PROBLEM WAS. It was astounding how they completely ignored the solution I repeatedly told them and kept asking for log files.
The Fix: tracks with unassociated files. I had a few dozen tracks missing the files due to deleting and renaming the actual file. I fixed every file with a broken link (by either relinking or deleting them) and sync works again.
This wasn't an issue on 6.8.0 so it's a bug in their sync programming probably brought on by the forced inclusion of Device Library Plus.