US Border: Is hiding data OK?

In most cases you won't have so much data to transport that you need to carry the physical medium, so concealing the physical medium solves the wrong problem.

Since Internet connectivity is widely available within the US, it will be a much more effective solution to encrypt your data with a good long passphrase that you remember, and then upload the encrypted data to a cloud storage provider or two, and don't carry the data physically with you across the border either in encrypted or plaintext form.

If they ask you to decrypt data, and you refuse, you will certainly be refused admittance to the US. The odds of this happening may be small, but it will depend on what the officer learns about you as you clear customs and immigration.

A good rule of thumb is that if you don't want it looked at, don't have it with you. Crossing a border is a vulnerable time.

The worst case scenario isn't likely imprisonment, but it might be a return to your native country (assuming you're not American) and a lifetime ban from entrance. (More likely it will be a return to your native country and a temporary ban, and greater scrutiny on future entrances after the ban expires.) It is up to you to decide how much you wish to push the boundaries of this.

The problem with that plan is that if they do decide to search you, they will look everywhere.

This is why attempting to conceal data makes it even more suspicious leading to an even more exhaustive search.

I say this in many threads, everything is subject to inspection, everything. It doesn't matter if it's a suitcase or a SD Card.

If you cannot risk border officers seeing it, you should not carry it across the border.

Keep in mind, unless it's illegal, border Officers are generally required to maintain any attached confidentiality. US CBP Policy described here. I'm willing to presume UK Border Force has similar policies.