iPhone music sync fails, and half of my music files are missing from iPhone

While syncing my music to my iPhone, iTunes says I have 1,374 music files. But my PC's iTunes folder actually contains 2,631 MP3s. What about the remaining 1,257 MP3s on my iPhone? I still have over 30GB of iPhone memory left.

As I ponder this question, after about 800 tunes are copied, i get this gem:

"The iPhone "iPhone" cannot be synced. An unknown error occurred. (-54)."

...then the 1,374 music files successfully finish copying from my PC to my iPhone.

I'm confused. How do I get my entire music library sync'd onto my iPhone?