Ableton Forum

I have no idea what I might have done, but a fresh install of Live on a new computer, and I can only arm one track at a time, both MIDI and audio. Any time I arm one of any of the tracks, the others are disarmed.

did I mistakenly set some option? This is frustrating & odd...

You can change this in Lives preferences, I think it's the record/warp/launch tab, but not at my computer to double check.

You can also arm more than one track by holding cmd while clicking on the arm button.