Hi all.
Juts in case anyone has the same issue, I was updating from a 3.2? version to 3.40 and it fell over during update (I copied up zip file using upload system files).
I had to use method of recovering of copying files manually to sd card and then upgrade firmware using M997 0 and then M997 1.
Error I got initially was
Error: M997: Firmware binary "0:/firmware/Duet2CombinedFirmware.bin" not found
The issue was there was no directory firmware on the sd card.
I created directory manually and copied system files and then was able to complete the tow M997 commands.

Hope that helps anyone who gets stuck.
Not sure if I did anything wrong, but new directory may well catch out people as it did me!

@phaedrux
Hi I uploaded the 3.4 zip file and after 20 mins the process had not completed. I power cycled and connected using USB, the 3.4 firmware had loaded but presumably not the wifi firmware as it was expecting it to be in directory called "firmware" which did not exist.
I only read upgrade notes quickly, but did not see any requirement to go from prior 3.? version to 3.3 first, should have looked closer.
All working again, just posting to try and put some info in case others make my mistake.
Thanks for fab product and support.

Hi all.
Juts in case anyone has the same issue, I was updating from a 3.2? version to 3.40 and it fell over during update (I copied up zip file using upload system files).
I had to use method of recovering of copying files manually to sd card and then upgrade firmware using M997 0 and then M997 1.
Error I got initially was
Error: M997: Firmware binary "0:/firmware/Duet2CombinedFirmware.bin" not found
The issue was there was no directory firmware on the sd card.
I created directory manually and copied system files and then was able to complete the tow M997 commands.

Hope that helps anyone who gets stuck.
Not sure if I did anything wrong, but new directory may well catch out people as it did me!

@jens55 I think it needs to use a wide angle lens or you will have difficulty seeing all the bed.

The nimble puts differing tension on the carriage depending on its curvature (forwards and backwards) and it's twist (left to right as cable leans over). I have now mounted nimble motor with 3d mount but that can also create problems. Suggest you disconnect nimble from extruder carriage and try a leveling run again to see what difference it makes.

Which carriage are you using as I have problems with my piezo reliability and cannot find satisfactory solution.

What cables and filament supply do you have to the extruder mount as varying tension and twist will affect z height depending on xy position. I have nimble cable which can give a similar effect. It look also like one of the y rods might not be parallel.