I have exactly the same problem. I have an external HDD (a Samsung original HDD that I just bought a few weeks ago for this purpose). I plugged it in to my Samsung SMart TV UE46D8000 - the HDD got recognized and formatted - and I strarted to record TV on it. Worked well. But a couple of weeks later I could suddenly not access the recordings anymore. The error message I get when trying to access "Recorded TV" from the Smart menu is "There is no device connected.".
Obviously I have tried everything. Disconnect / reconnect. COnnect to different USB ports. Turn off TV completely, wait a few minutes, turn on, then connect the HDD. I have even reformatted the HDD on a PC (losing my TV recordings), and then reconnecting it to the Samsung TV in hope that the TV would now re-discover the HDD (as it did the first time a attached it), and give me the option to format it again. But nothing...
I am sure that the problem is in the TV, since the HDD works fine when I attach it to my computer. And the light on the HDD is on when attached to the TV, so it does have power.
Samsung, please help!