Fortunately, this bug is more of an inconvenience than a real hindrance. Some people have reported music mysteriously disappearing on the iPhone and iPod Touch, right after updating to iOS 4.2. The music still appears on the iOS device when connected to iTunes but does not show up on the device itself. This appears to be a database bug, as the music files are still actually on the iOS device, but you just can’t access them. Fortunately, the solution to this problem is quite easy.
Then in the sidebar, click the arrow next to your device and select music.
Play any song and sync your device again.Happy iOSing,
iWolfy
http://www.twitter.com/iWolfy <-- well worth a follow
