attempting to copy to the disk failed. an unknown error occurred (-54) external drive

Starting this morning, I get this error when I sync my iPhone:

Attempting to copy to the disk ‘Media’ failed. an unknown error occurred (-54)

My iTunes media is stored on an external drive (called ‘Media’).  I have tried:

• repairing disk permissions
• repairing the disk ‘Media’ using Disk Utility
• restarting the computer
• doing a hard reset on my iPhone

There seem to be a lot of threads discussing this particular error, but I haven’t seen any solutions that work for me.  Has anyone else had this problem and solved it?

Thanks!