Thanks to all the contributors, their help proved extremely useful! I found this thread searching for a solution to upgrade two boards that refused to be recognized by the mission planner firmware update tool.
I have no tools to know why this happened but an entire day of trials did not solve the problem, bootloader update didn’t work and anything i tried was vane. I am writing now because i have been able to update both boards (a 2.4.8 with 2 mb and a hobbyking px4 clone) in 15 minutes just using qgroundcontrol…without having ever been using it before one hour ago.
Launch qgroundcontrol, connect board, pressing firmware update
QGC asks to disconnect the board and reconnect it
Upload px4 firmware (not ardupilot)
reconnect the board, wait for the parsameters loading and then go to parameters
search for the parameter sys_bl_update, set it to 1 (enabe)
disconnect and reconnect the board, this will update the bootloader
At the end the board reconnects automatically, wait for it
Disconnect the board and reconnect, before parameters are loaded you can press firmware
Load latest firmware, using fmuv2 option, try fmuv3 if it doesn’t work BUT this time set ardupilot firmware.
Firmware will load, once finished disconnect everything, quit qgc and you’re ready to connect a firmware-upgraded-board to mission planner.
Don’t know why firmware update with mission planner went so difficult, it was fast and straightforward but now, surely because i must have made some mistake at a certain point, was impossible.