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> So searches of digital content are at least outside one of the key purposes of the border search exception.
Yes. And also, who in their right mind would use a phone and an airline ticket to smuggle at most 32GB (or 64, or whatever) of contraband data into a country? Absolutely no one.
1. my phone has a 200GB microSD card in it right now 2. 256GB microSD cards are readily available 3. phones with two microSD cards are available 4. I can easily purchase a 4TB hard drive that is roughly the same size as a pack of cards
It's not terribly relevant, but you're dramatically underestimating the volume of data that can be reasonably stored by media carried on one's person.
4) Encrypt that drive and ship it. Why would anyone up to no good go to the bother of passing through border security with it on their person?
Your phone is undoubtedly your "effects" (in 1789, touching your phone without your consent would have been a trespass to chattel). But searches incident to a border crossing have always been considered not "unreasonable," because inspecting the flow of things across the border has always been seen as a legitimate government function. Indeed, one of the very first things Congress did in 1789 was to create a customs department in order to perform precisely such inspections.
That said, there is an originalist legal argument to the contrary. The framers understood the border-search exception to exist for purposes of customs enforcement. Customs enforcement has always been about physical, rather than intangible goods: https://www.eff.org/files/2015/11/10/clearcorrect_v_itc_-_op.... So searches of digital content are at least outside one of the key purposes of the border search exception. (Of course, that doesn't help with the other purpose of the exception, which is national security.)