I have had this happen to me a few times on my current and last couple macbook's.
So far no serious problem, but it really worries me when sometimes the computer does not recognize the hard drive...
Usually it shows up very quickly; but once in a while, it just does not show up on desktop or finder. The hard drive is powered on and the disk is spinning, but no file.
So I have to unfortunately just unplug it to power it off (I do not get a message for incorrect removal) and then re-attach and it comes up after that.
Anyone have this problem or guess what it might be?