The following note is taken from the OpenWrt Table of Hardware page for Ubiquity AirMax M devices.

Ref:   http://wiki.openwrt.org/toh/ubiquiti/airmaxm


In summary, if you flash a Ubiquity XM device manufactured after July 2015 with OpenWrt you **WILL** brick the device!!


Special Firmware Note: 
----------------------
AirOS XM.v5.5.X images used U-Boot 1.1.4.2-s594 (Dec 5 2012 - 15:23:07). 

The OpenWRT image can be successfully flashed onto these versions of firmware. 

However, in July 2015 Ubiquiti released a new version of firmware XM.v5.6.X. 
With this firmware a new U-Boot version was released, U-Boot 1.1.4.2-s956 (Jun 10 2015 - 10:54:50). 
The newer U-Boot version changes the memory size and starting address for rootfs, cfg, and EEPROM. 

LOADING AN OPENWRT IMAGE ON A U-Boot 1.1.4.2-s956 WILL CAUSE THE DEVICE TO BE BRICKED!!!


If the device has XM.v5.6.X, an older version of XM firmware can be loaded from the AirOS webgui (for example XM.v5.5.10) and U-Boot will be overwritten with the older version. OpenWRT can then be loaded onto the device successfully.