Same problem for me
I'm having the same issue with my UA55D7000 and a 2TB Western Digital 'Elements' USB HDD, which has about 1.5TB of content on it - the content hasn't been recorded through the TV's PVR function, so it hasn't been through any formatting process with the TV, it's supposed to be just 'plug and play'. The HDD is powered by it's own power cord, so it's not reliant upon the USB connection for power.
When I first started using the HDD, the TV had no problems recognising it (it took a little while when it was frst connected) and even after turning off the TV (say at night time), when it was turned back on again, the HDD was still 'remembered' and was available under the list of source inputs.
Now, the HDD doesn't show up at all in the source list. After some stuffing around (turning the TV off, then on again and waiting for about 10-15 mins), the TV 'recognises' the HDD and a message pops up oin the TV advising that the HDD has been connected. But, once the TV is turned off using the remote, the next time it is tunred on again, no USB drive.....
So, I know that the HDD works on the TV because a) It used to work fine and was recognised quite easily and even remembered by the TV, even after being turned off; and b) It is still working now and is still recognised, but it takes a lot of stuffing around to get it to work, which is a real pain.
Hopefully there is a simple solution out there?!?