The chances of your electronic device being searched at the boarder is low. In 2018, only about 33k people had their devices searched out of hundreds of millions of people entering the country.

The answer to your question is fairly simple, and does not matter which type of disk you are trying to cross the border with (and you do not allow CBP to see unencrypted data). (if the CBP attempts to search the disk)If you are a US citizen, you will eventually be allowed to enter the country, although you may first be detained. If you are not a US citizen or permanent resident, you will probably not be allowed to enter the country. CBP may make an image of the disk to try to decrypt it off site or possibly at a later time (for example if weaknesses of a particular encryption algorithm is discovered in the future). There is no law against carrying encrypted data across the border, and no requirement that you disclose any decryption keys.