Same issue for me UE46D7000
I have exactly the same issue.
My UE46D7000 has been reading .avi files perfectly for months off my external hard drive (Western Digital 2TB), as well as a Kingston 8Gb thumbdrive, but a few weeks ago, for no apparent reason, it cannot browse either drive. Both drives appear in the list when I press "source", but as soon as I select either drive, it just goes back to the HDMI input instead.
Am I entitled to exchange the TV because this feature is obviously not working for lots of people?
It's not down to the size of the external drive, because it's the same problem with the 8Gb thumbdrive as well.