Like many i was not able to upgrade my Keystep firmware.
Here's the solution i just found using the original USB cable.
Maybe it helps others.
• Quit Midi Control Center and Audio Midi setup, if any are open.
• Disconnect Keystep and download latest firmware first.
• Start both Midi Control center and Audio MIDI setup.
• Delete "Arturia Keystep" entry in Audio MIDI setup / MIDI studio, as well as any generic "USB MIDI Device" entry and/or "KeyStep Updater" entry
(Note, that a connected item cannot be deleted).
• Connect Keystep to USB while holding REC/STOP/PLAY down.
You'll see the Hold, Shift and Oct+/- buttons illuminate a rotating circle. This says BOOTMODE.
• Now follow the steps given by the Midi Control Center. Use locally downloaded firmware copy.
Worked here like a charm. *Enjoy.
