I’ve personally seen this issue on my retina MacBook Pro — what I thought was just a faulty hard drive is not. I hope this is a software problem and not hardware but it is an issue I am seeing as well.
If I plug in my desktop HDD or USB 3 memory card reader it is rather hit and miss as to whether the computer will mount the devices. It’s less than 30% of the time right now. I have tried different cables and ports on the Mac — nothing matters.
However the mobile LaCie USB 3 drive I have mounts 90% of the time. A bunch of people in forums seem to think that this is related to a low bus power that Apple is putting out, but I tend to think it is more software related.
So buyer beware for now.