I destroyed my machine and had to get a new one, and reinstall everything.
I installed a new copy of Android Studio.
I created a test project. I compiled and ran it on my tablet.
I also use Processing. I set up Android mode in Processing. And told it to use the SDK that had been installed with Android Studio.
I compiled a Processing sketch and compiled and ran it successfully on the attached device.
I now open an existing project that I was working on on my old machine, and copied from my backup disk onto my machine.
Open it in Android.
And the "Loading Devices" drop down is empty. Android can't see my connected device.
I create a new project again. I get a lot of messages saying Gradle can't find stuff, and it downloads a lot of files. But in the new project, my tablet again appears.
Go back to my old project. Still greyed out "loading devices"
So ... why is this happening? I presume that the older Android project can't see the device because of some older, outdated configuration is blocking it. But where is this? Which file? And how do I recognise it?
And why should Android Studio's ability to see devices be Project dependent anyway?