Then it should be secured in a safe or encrypted.

This is part of "old-school investigating". The dog is to help them execute search warrants. The child porn can be stored on any kind of electronic storage medium, and that can be hidden pretty much anywhere in the house. It's a ton of failure-prone work to dig all that stuff up so you can search it.

In this particular case, it actually involves undercover work, too. Investigators get on P2P file sharing networks or infiltrate underground trading rings (which is sometimes pretty tough) and find people trading illicit material. Often, judges want a fair bit of supporting evidence that they're intentionally sharing explicit material (since everyone knows the "a virus did it" defense), so they'll get the target to reveal information sufficient for a warrant. (On top of that, they have to make sure the person is within their jurisdiction.)

Often times, a child porn case starts because someone calls the cops, and that requires a fair bit of proper investigation, too. Usually the accused is in contact with a child, and you have to figure out if something is going on there. Sometimes it's people planting evidence to get back at an ex-boyfriend or something, and you want to eliminate that possibility, too. (One guy tried to steal his neighbor's wife by planting CP on his neighbor's computer. Really not a great plan.)

"Storage" is what, these days, we call I/O-based secondary memory. It's still a form of computer memory, though.

You need a photo ID and a boarding pass to pass the checkpoint, and they record it when you do. The area is under video surveillance. It seems like they have tracking pretty well covered.

Oddly, I could swear that this has been a theoretical travel rule for ages (at least, before the TSA existed) -- a security checkpoint "may" ask you to power on a laptop to demonstrate that it's really electronics. No idea what happens otherwise. I don't recall ever encountering it.

Most of the people they actually catch and prosecute are pathological collectors.

Rhode Island is actually a little unusual in that they're pursuing people based on online leads. That's a ton of work. Last I knew, most state forensic labs already had their hands full with evidence to process from direct-referral cases. Those are were someone calls the police to initiate the investigation. Those cases are easier, since there's independent (and non-digital) evidence or testimony. They're also very often associated with actual abuse of a child of a friend or family member (whereas the guy you pick up on the other side of a P2P file sharing network could be otherwise harmless). So most places don't bother pursuing online leads, because they already have their hands full with easier cases.

They're not looking for (or claiming) "certain content". When you get warrants to arrest someone for child porn and search their house, the search generally includes seizing any digital media on the premises. Reasonable, since the guys often hide their incriminating collection somewhere. Digital media is small, easy to hide, and comes in all sorts of forms (as you well know), so reliably finding all of it in a house can be a real pain. Guys have gone free because the police didn't find the incriminating drive during their search, and the guy had a friend wipe the incriminating drive after he was arrested.

Seems plausible that a dog could sniff out electronics, which is really all they're looking for.

The dog is owned by the Rhode Island State Police, who don't do border searched. The regular police still need search warrants in the "buffer zone".

The mistake of saying that it contains evidence and that he has the capacity to unlock it is a huge mistake. In general (IANAL), in order to compel you to do something like unlock a safe, the prosecution needs to have a reasonable belief that the safe contains evidence relevant to the case, that you know how to unlock the safe, and that the safe (and/or evidence?) are yours.

He admitted all three of these. If he hadn't, then at the very least they'd have to work much harder to prove to a judge that all three were true before trying to compel him to decrypt the drive. The latter two are probably easy -- if the drive is in his physical possession and attached to his computer, then it's reasonable to assume (though not always true) that it is both his and that he has the means to access it. But they'd need something more than a hope or a guess that it actually contained evidence to compel decryption.

The word "witness" does. A witness is someone who provides testimony in a legal proceeding. It is not someone who provides non-testimonial evidence relevant to a legal matter.

...is not the way the law works. Testimony is separate from other kinds of evidence, and the 5th Amendment covers testimony. (The 4th Amendment provides different protections to other kinds of evidence.)

Despite how frequently it occurs, it turns out that your home-grown, intentionally-overbroad, untrained interpretation of the Constitution is not, in fact, a good basis for reasoning about the law.

A good starting point would be not telling the police the contents of the encrypted hard drive before decrypting it (as he did). Now they have a pretty good guess as to what the drive ought to contain.

In two ways. First, being compelled to provide evidence that already exists isn't protected under the 5th Amendment, only giving testimony. (There are lots of details here, but that's the short version.) Second, once you volunteer information, you waive your 5th Amendment protections for that line of inquiry. (Again, details.) He told the police that the encrypted drive contained evidence and that he had the ability to decrypt it. Not a good move.

Easy. For changing your password, at least, passwords are transmitted to them in cleartext and hashed server-side. Hashing passwords is done before storing the password, not before transmitting it.

Or he knew how to use negative numbers.

Or, more than one part in 10^6, which is larger than our measurement precision in some cases.

Relativity only actually requires that particles with mass stay on one side of the speed of light: strictly below or strictly above. The "strictly above" particles, which are entirely theoretical, are called tachyons.