DON'T SWITCH YOUR PRINTER OFF AND LEAVE SOMETHING CONNECTED TO USB
I have 2 almost identical setups - Creality Ender-3 Pro with BigTreeTech SKR Mini E3 V2 mainboards, BTT UPS, BTT Relay, BTT Smart Filament Sensor and PEI build surface - each with a Raspberry Pi 3B+ running OctoPrint. The only hardware differences are: one has a 0.4mm nozzle, original extruder and a BL-Touch Clone; the other, a 0.3mm nozzle, BMG clone extruder and an original BL-Touch. The custom Marlin firmware is idenical apart from a few measurements specific to each machine - E-steps, offsets, and such like. The printers are each contained in a small Creality enclosure and have been working normally.
Last night I just switched off the printers at their power supplies and left each of the Raspberrys running. The screens of the printers remained dimly backlit and I thought nothing of it. This is not the first time I have done it.
Today I have gone to switch them back on and there was an unusual smell as I opened the enclosures. No sign of a problem otherwise. I know I moved the Z-axis up on at least one of the printers so that the BL-Touch wouldn't be damaged and so that it would home correctly first time.
I went to the other room and tried to print something and as the printer tried to home their was a loud rumbling/"grinding" as the Z-axis was lowered. The homing failed and an emergency halt was shown on the relevant OctoPrint. I went and tried to fix it, switching off the printer and disconnecting the Raspberry Pi before switching it back on. The same problem occurred. I then tried the other printer and the same problem.
The only other difference is a recent update/upgrade to OctoPrint.
My guess is something to do with Marlin being setup to hold the Z-axis to avoid dropping down and damaging prints or the print surface. Has the motors been damaged? Or has the drivers for the Z-axis on the mainboards been damaged? Has the mainboard been killed?
The smell would suggest something electrical/electronic. Power is obviously being provided over the USB, but not enough to run the mainboard and display at full power, let alone the motors. Maybe something to do with the current to keep the motor from turning?! I am not in a position to test this moment as I had left it too late in the day before going to print something, as such I will be off to bed feeling rather depressed!!!
Luckily I have a Creality 1.1.4 board that came with one of the printers. Hopefully this works as it was never used by me. My first printer came with the silent 1.1.5 board but the Z stepper motor driver on that one died and needed replacing - so I decided to buy 1 more printer and 2 of everything else I wanted so that there wouldn't be a total outage if one of the printers went down - didn't expect both to go!
I don't expect OctoPrint to be the culprit but if someone can check. In the meantime I would suggest that anyone, at least with a simililar setup, disconnect the device running OctoPrint from their 3D printer BEFORE switching the printer off. Ideally I think this information/warning/advisory notice needs to somehow go out to all users, just in case.
I will update here in the next day or so. It will take more time and effort as I have a number of disabilities (going on 11 and a half years since getting Swine Flu H1N1 in 2009 - if only the bloke across the table had been wearing a face mask/covering back then!!!!). If someone else has any experience or help, let everyone know below. Also....anyone got a compatible mainboard or 2 going free?! I'm in the UK and could only afford these setups because of lockdown, being classed as extremely vulnerable and thus not having to fuel our vehicle as often.