this. Use the whole system encryption and carefully use the plausible deniability feature to dual boot. The decoy system must be reasonable. Add some files, especially if you need them on your trip, browse the web, do searches for holidays in the US and add some porn. Seriously: You are suspicious if you're male and there is no porn on the private laptop. Combine algorithms when doing the whole system encryption, just in case.
So. At the border, give them the password for the decoy system. It should be reanonable complex password. If they take away the laptop for one minute, consider it to be compromised. Do not use it anymore or only use the decoy while being aware that your activity is closely monitored. Do not get your into your more private emailaccount abroad, only use the garbage account provided by an american company. Do not login to anything remotely security related, just as you would on a public terminal.
Or you do a clean install and take a HDD with your backup in a hidden container on the hdd.
Oh. And your iPhone or Android MUST be factory reset before entering the US border. Once they have physical access to it, they can extract the information, including important password stored on the phone. The backup could be on your laptop, the cloud is obviously not an option for a backup.
In most cases, nothing will happen. But be aware that you are entering a police state that is only a few years ahead of europe. So you came prepared.
To the FUD-spreaders in this thread: Snowden himself trusts true crypt enough to use it. Some enryption is compromised, but not all. If you combine different algorithms, chances are high that the decryption might take enough time for a mortal being to have it decrypted when it doesn't matter anymore.
Spreading FUD leaves Joe Average, who might get the whole system encryption installed with some help with nothing he can do. And that is not the case.
Yes, the tin foil hats were right. But even if everything is pretty much fucked, all is not lost. Personally, I prefer fighting back instead of doing nothing because the others have already won. Because they haven't - quite yet.
If you are a target, you're screwed. That is true. But at least the peeking into your whole privacy at a random custom search like this is deflectable.
After returning from the US, either throw away the compromised laptop if the laptop was taken away even for a short time or flash the BIOS, repartition the HDD, reinstall the operating system and hope for the best (that the bug was only planted in the BIOS and the mbr, not in other firmwares and that the bug in the BIOS was overwritten by the flashing).
If the laptop was not seized, simply decrypt the right system and repeat the steps above with a more perfomant combination of algorithms. Of course, all these steps are moot if you are a target.
BTW: Consider all closed source operating systems to be compromised big time. Only a few people need windows as the first operating system anymore. Most people will be fine with a restricted virtual machine within a Open Source OS.
Gamers could do a dual boot to play and reboot for work and browsing, but that is very inconvinient. At least until newer games are brought to linux by steam. And here is the problem of the closed source steamclient and the closed source driver for the graphic card, but my guess is that strange behaviour would be easier to find by the community on linux than on windows, so the attack vector is still smaller.
But still, if you are a target, you are screwed. The russian secret service has ordered hundreds of type writers recently for a reason.