I've experienced a very similar issue a couple of times.
I'm also using a Mac (Snow Leopard) to copy my music and charge the i9. I found the i9 to be very
sensitive to « unclean disconnections ».
In other words, once you've plugged your i9, do not unplug it until you've hit the eject button
in Mac OS and the player icon has disappeared from your desktop.
Every single time I forget to do so, and even when I put my MacBook to sleep by closing the screen, well ... I hit the same problem you do. No audio file is visible from the i9 and a hard reset is soon required.
When this happens, I necessarily have to format the device again ...
If someone has a solution to avoid formatting, I'm all ears.