I've had a subset of Mikrotik 5G LHGs and SXTs (they use the same motherboard) load the rb.elf file but refuse to take the second step and load the sysupgrade.bin. I posted and got a lot of help here (links below) but no solution.
https://www.arednmesh.org/content/mikrotik-router-boards#comments
https://www.arednmesh.org/content/flashed-9-lhg5nd-us-nodes-7-worked-and-2-refused-conversion
I've spent a lot of time on this and believe I've figured out the problem. 5 out of my collection of 6 nodes that refused to cooperate are now happily running Aredn. A couple weeks ago I ordered a 3 pack of LHG 5nD (from the very big online store) and all 3 refused to go beyond the rb.elf step. That was the last straw. With days of experimentation and help from the Copilot / Grok AIs, I learned it is a problem with a chip on the motherboard overheating (likely the SoC). Sandwiching the board (in a baggie) between 2 frozen plastic ice blocks in a cooler allowed the flashing process to complete. A spritz of liquid from an electronics "freezing" can on the SoC temporarily helped buy did not last very long. The frozen blocks were better since they provided continuous cooling.
LHGs are remarkably easy to disassemble and the motherboard-antennas complex is easily removed. In Mikrotik RouterOS I set the system-routerboard-settings switch to "try ethernet first" then exited RouterOS. I put the board in a baggie with Ethernet attached, sealed the baggie as well as possible and put it between the frozen blocks in the cooler. I let it cool for a few minutes. Upon powering up, the do-read file loads from the PXE server and the flashing proceeds normally.
The downside is that this is just a work-around, the problem is not solved. I may need the coldbox treatment to update the Aredn firmware? The AIs suggested possible "cures". Grok speculated that the last 3-pack of LHGs were perhaps refurbished and this is commonly seen.. "It" suggested replacing the double-sided heat-conductive tape between the SoC chip and its heat sink with 3M 8810 thermally conductive adhesive tape. I might add a bigger heatsink. I'll add comments to this post if those actions help.
Edit: 9-26-25, well, the "better" heat transfer tape didn't help much, and indeed, upgrades/downgrades still need to be done in the cooler. However, they work fine under those cooler conditions. I had to use the "dangerous upgrade" switch a couple times when I got the "firmware-node mismatch" error. At least I can use these nodes rather than throw them away. I post this to help anyone who runs into a similar situation.
Orv W6BI
Orv W6BI