I'm having problems with this too. For years I've been using NetInstall on several different RB types with no issues. Now however, I have a batch of RB433 boards coming in with ROS 5.26, firmware 3.18, and all of the half-dozen boards I've tested so far have refused to NetInstall as expected. I don't get the "NAND bad" message that David is seeing; in fact I see *no* serial output at all... which is preventing me from selecting the appropriate U-boot options to boot from NetInstall.
Since I can't get these boards to PXE boot, they boot to the installed ROS, but come up with a 0.0.0.0 Ethernet address. (This is no doubt why the BOOTP push button also fails to connect to NetInstall.) After playing around with them quite a bit (downgrading ROS, attempting to downgrade the firmware, using the BOOTP push button, etc., etc.) I have eventually been able to get all six boards to NetInstall but only after considerable effort with procedures that have varied from board to board. Once reinstalled with ROS 5.4 (and retaining the 3.18 firmware), they seem to function normally and the serial console once again displays as it should. The problem is the inconsistency of the steps it has taken to remedy these boards... and I have 26 more to do.
I'm pretty certain we're seeing a bug here. Normis, any suggestions? Is anyone aware of any work-arounds or do I need to send these back to the distributor?