They can image the device, or possibly even refuse it entry if they can't read the contents, but they cannot detain, harass or otherwise coerce the owner of sai

They can image the device, or possibly even refuse it entry if they can't read the contents, but they cannot detain, harass or otherwise coerce the owner of said device while it's in the mail. Mailing an encrypted hard drive is likely reasonably safe with regard to unauthorized access to its contents.