The AR750S-EXT has had several issues with uboot, as there was some changes made along the way with both uboot and the firmware that sometimes requires that you have to step up to a new version of firmware. I have only had luck using uboot on my AR750S using firmware 3.104. From there I use the GUI to update first to 3.105 with the tar file, then to 3.2xx with the tar file . I have no clue on what will happen when the 4.x firmware finally is available.
Here is one recent post on how someone else finally got it to work:
There is a second known issue if you have an older version of the AR-750S-EXT where you can only get uboot to work with Windows. See:
It is very unfortunate that there has not been a clean and easy way to use uboot to update the AR750S firmware since the release of the 3.105 code.