Even i have the 1TB hard disk external HDD and it gets detected on my LA40B650 without any issues. I feel the USB port current rating of your external HDD box may not be compatible with LN40B650. Check the ratings. i hope changing the external HDD interface box would resolve your issue.
I have a question about my LN46B650 that's been bothering me and I'm wondering if anyone on here could give me some help with it. I've used the TV's USB functionality a couple times with my 4GB flash drive and it works perfectly. The TV recognizes the flash drive, let's me browse movie, picture and music content on the drive and I can play it no problem. It plays AVIs and MKVs as advertised and looks great. Since I have a bunch of those files on my 1TB external hard drive (Cavalry CAUM Series 1TB drive, http://www.newegg.com/Product/Product.aspx?Item=N82E16822101121) I figured I'd just hook up my external drive to my TV so I can browse and play files off of my drive. When I plug the drive into the USB port, the TV doesn't recognize the drive and the TV says there's no content available to browse.
Am I doing something wrong? Is there a reason my TV recognizes my 4GB flash drive but not my 1TB external hard drive? They're both on USB 2.0 devices so I don't understand why the TV would recognize one and not the other. The external drive DOES have it's own power supply, does that have anything to do with it?