Hey there, I am workin w/Droid & got same message after transferring songs to the storage card. When I started the operation, I had to choose "Mount" the storage card. sooo, I thought I needed to select "Turn Off" to unmount. This resulted in the error message "Sorry, the storage card is busy" and it showed us as "unavailable" on the settings menu.
THE FIX? I reconnected the phone to the PC and found the "USB" connection tab on the phone. Then, when I selected it, it gave me the option to "Mount" ... I did & waahlaahhh! My music became available on the phone once again.
Hope this helped.
Happy Happy and closing my eyes to listen to Jimmi do "Red House" again (-: