Fixing Asus UX21e unexpected power off
The Asus UX21e (and maybe the UX31e?) has the irritating misfeature that it reloads the CPU thermal tables when you unplug the power. One consequence of this is that it'll automatically throttle itself much more aggressively on battery (reducing performance) but the more serious one is that the new critical power off temperature may then be lower than the temperature the CPU is currently operating at, resulting in the machine turning itself off. As far as I can tell from debugging, this is completely OS-independent - it still happens even if I stub out all the ACPI code for power supply events and there are reports of the same thing occurring on Windows. The good news is that it seems to be fixed in newer firmware versions. The even better news is that you can flash it without Windows. Just download the BIOS image from the Asus website, copy it onto a FAT formatted USB stick, insert that, go into the firmware (hit F2 on the splash screen) and start the flash program from there.