I also have an LN40D550 and never had problems with my 1TB HDD, but I use an AC adapter to power it. I know you're using thumbdrives now, but you could also look into a USB cable with an extra connector that plugs into two USB ports to give the HDD that extra juice it needs.
I don't really use the drive with the TV though, it's easier just to just connect the TV to my router via ethernet and just stream the movies from my PC. Only issue is it will play .MKV off HDD, but the exact same .MKV files won't play via streaming, but you can rename .MKV to .AVI to fool it and then they play on the TV just fine via streaming. Clearly just a bug in the TV's firmware that they never bothered fixing.