One of my internal drives (not a boot drive) sometimes will just not mount and is not visible in the Disk Utility. Sometimes when waking the computer from sleep, it will give me a message in the top right saying something about it (I think that it is not unmounted properly or something, but I can't remember exactly). Usually it will come back after a restart, sometimes not. I don't know if the drive is failing but it runs fine when it is mounted and doesn't ever disappear while the computer is running, only when it has been asleep. Any tips?