Can't eject my iPod

Often after copying songs from iTunes to my iPod mini and pressing the eject button, my computer tells me that "this device is in use and cannot be ejected - try quiting applications". So...I quit iTunes and highight my iPod and select eject or drag the iPod icon to the trash and get the same message. What's going on, and how can I safely disconect the iPod when I constantly get these messages? Thanks.