Is the update the only file on the thumb drive?
If you have a lot of other files on there the TV can get confused and not find the file.
For some reason, my UN46B8000 will never accept a a firmware update through the USB port. I follow the online directions verbatim, yet it says no software is ever found. It has been this way with all versions.
Also, upgrades are always available by download through Samsung's website way before the TV will find it online via the Ethernet connection.