A border agent would just open that bag right in front of you, making it not a particularly useful measure of being tampered with. I suppose a large amount of

A border agent would just open that bag right in front of you, making it not a particularly useful measure of being tampered with. I suppose a large amount of the problem could be solved just by taking checksums of all non-volatile memory on the device - however that doesn't check for, for example, hardware keyloggers which might be inserted without your consent, and then a thorough evaluation of the hardware would be necessary. However that still doesn't tell you if somebody has simply tried to copy data off of your device - so maybe in this case you need something which physically marks the device in the case that the hard drive is removed and presumably accessed outside your computer, like those dye traps they use in banks and when transporting money.