The only place that I've had this issue was connecting through Istanbul. It was weird to wait for them to see it boot. They were fine with seeing a bios screen.

There’s a ban on fake laptop? Why does the TSA care if they are real?

Presumably if there's a working computer inside the case then it can't be packed to the brim with c4?

Yeah, someone would have to leave a Raspberry Pi zero sized chunk of explosive out to put up a boot screen and something close enough to a windows boot logo to pass a cursory visual inspection.

In reality, the X-Ray should already make it pretty clear if there is something weird about the laptop's build. Turning on the machine is entirely unnecessary. Also, I have not been asked to turn on a laptop in many years, so unless this is a new development it is badly out of date.

The metal casing of many laptops reflects the xrays, hence you can't see anything inside them.

At many airports you can see the x-ray images of the bags and devices as they are scanned/checked.

In my experience there is always a detailed image of the inside of laptops. Color-coded according to material type/density and clearly showing the location of batteries, etc.

I'd imagine the batteries being switched for plastic explosive would be a simple way to take explosives onboard for unsophisticated attackers, turning on rules out that possibility. Not everyone who wants to blow stuff up is a hacker.

Wouldn’t plastic explosive have a different density to real batteries, and thus show up very obviously on the scanner?

The machines are tuned to highlight specific risky materials (such as liquids & gels).

At one point it was common to be asked to turn on laptops etc. But I haven’t seen them do that for years - presumably because the scanners have improved.

LiPo laptop battery is apparently around 1.8-2 g/cm3 (sorry can't find good figures), whilst C4 is 1.73 g/cm3. Not sure what the density resolution is like -- I'd guess it's poor because you can't tell what thickness/type of casing you're looking through from the luggage "xray"; I'd imagine it to be primarily good at relative density.

X-Rays can pass through thick chunks of metal several times the thickness of a laptop. You just need a powerful enough source. I doubt however that such a source can be used safely in an enclosure as open as the securiry check machines.

Not even close to true - just look at the scan monitor next time you fly: you can clearly see inside any electronics going through the scanner

I had a slightly damaged laptop with a loose flap on the bottom and I got told off by the agents somewhere... Syracuse I think...

Never had any issues with that laptop anywhere else - but they got pretty grumpy at me, and made me power it up to prove that it was a legit laptop.