In order to discover it was child porn they would have to physically open a file; it’s not like you can look at the file’s bits in a hex parser and determine that an image is in fact porn.
As for the other analogy, if your financial records were on the drive they would have no reason to open the files unless they were snooping around.
As for the “cocaine/porn in your trunk”, the analogy is flawed. The violating item is in plain sight in your trunk. The proper analogy to put this in perspective is that the item is in your glove compartment, the mechanic is fixing the tail light, but goes and opens the glove compartment to see what’s inside.
I would have a number of questions not only for the mechanic but for the owner of the business in this situation, and as such would have a number of questions for any person who effectively opened the glove compartment on my drive.
I surmise that you've never used the "thumbnail" feature on your computer's file manager.
I still don’t think you have any expectation of privacy when you willingly give the information to someone else.