I disabled the internal screen, then I unplugged the external display…… and I got a blue screen, and I had to turn my Mac Off to get access again.
This is unfortunately a bug in macOS. When the internal screen is disabled by SwitchResX, you have to turn it on manually before unplugging the external screen. macOS will not enable the internal screen automatically.
Worse: it will think the internal screen IS active, when it is actually not, so any tentative to force turning is on will be unsuccessful...