OS X no longer sees my DVD drive. How to debug this? My Mac Pro has its original built-in DVD burner, which has always seemed to work fine in the past.  Sometime in the recent past (10.9.1?), it simply disappeared.

I first noticed that pressing the eject button on my keyboard didn't do anything.  I looked up alternative ways to eject the tray, and I found the "drutil" program, which seems to confirm that OS X doesn't see any drives at all:

$ drutil list
   Vendor   Product           Rev   Bus       SupportLevel

$


I've rebooted, and that doesn't help.  I don't even know how to debug this.  Where do I start?
I rebooted a second time, and now it works.  Crazy.