Boot failure?

I have raised a helpline ticket for the following, but thought I might put this to anyone interested:-

6600M, iMac Pro, and SmartSDR for Mac smart control.

For weeks, this combo has worked very well for me. Yesterday, when I turned on it my usual way, I got the thrice-blinking led, and no boot-up. I shut down, disconnected the ethernet cable, re-booted and all was well. i.e. the free-standing rig worked (thank goodness for the "Maestro" front panel built in). I found that when I then connected up the iMac ethernet cable, I was then able to then move over to SmartControl quite happily. Weird or what? I have checked alternative cables, fully pulled power, waited the prescribed minutes of wait-time before re-connect, etc, etc, etc.... I get the same result. Closing down and the restarting in my preferred way, and back came the blinking reds. So the only way I can get SmartSDR Smart control is to disconnect iMac, boot up the 6600M, then hook up the ethernet cable after I have got to the screen for "RUN/SMART CONTROL" and then select accordingly.

I noticed that I did not hear the start-up fan-speed-test during attempt at boot-up which resulted in red-leds, whereas when I booted without the ethernet cable attached, the fan-speed-test did occur as you would normally expect.

Has anyone else had a similar problem?

Cheers. Mike. G4GWH