OS X Yosemite Improperly Ejecting External Devices
It happens occasionally with my external drive which I suspect is a connection problem. I run 10.7.5 so I would judge it unlikely to be a Yosemite issue. If slightly moving the cable around replicated the behaviour, that's probably it.

But no, clearly this is not normal. It would not hurt to try it on other systems which I don't currently have the hardware to do.