ModemManager respawns automatically when you kill it, thanks to SystemD.
Your best bet is to use systemctl disable ModemManager or better yet, apt-get remove modemmanager 
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I had similar problems with the firmware update. At the end I was able to update the firmware with the ST-link-V2. You can find the tutorial on the odrive guide. To make it work I did not follow the tutorial in every point, if I remember correctly I had to connect the Vcc between the St-link and the Odrive. I’m not shure of it, so verify my words before trying this, I don’t want to be the cause of a hardware failure. Hope this can help you.
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