If you wrote it down in a journal then you no longer have the right to claim 5th amendment privileges on what was written. Why would it be the case that what you put into your smart phone is more protected than what you write in a book?

http://lawdigitalcommons.bc.edu/cgi/viewcontent.cgi?article=...

Excellent, that is exactly the kind of answer I was looking for and it seems the link you mentioned above almost directly applies.

I think they would view that as destruction of evidence.

The next step in your scenario is "that security camera has data on me, I want it deleted".

Not really, especially if the cameras are 3rd party owned. My comment was explicitly about my own devices, with the argument being that they are an extension of my brain/memory as opposed to evidence in the traditional sense.

The link provided by wang_li applies and it seems such questions have come up in the past in front of courts, and honestly I won't be surprised if they come up again.

I believe GDPR does cover the security camera scenario, there was a comment here about a week ago from someone who successfully petitioned the German police to delete all footage of him at some protest

https://ico.org.uk/your-data-matters/cctv/ at least in the UK, you can request people who have CCTV footage of you, and also note that people have to delete CCTV when they no longer need it (how long that should be is arguable), so people have to delete CCTV without being prompted to. and this is not new with the GDPR, it's from the old law, from 1998, I think.