I landed an hour long interview with a senior developer and am pretty nervous. I know it's going to be technical, but is it appropriate at all to ask about what I should expect?
Whether or not it is appropriate to ask what the interview will cover is going to be very dependent on the interviewer's opinion. It could go either way:
The interviewer could be bothered by this question. It would be fair to assume that the interview questions will cover whatever skills were listed in the job posting and what you've put on your resume. The interviewer could assume you don't know those topics as well as they want you to if you feel the need to ask what will be covered. It's kind of like asking a professor what will be on a comprehensive final exam (the answer is usually, "everything we covered in the course").
Other interviewers will not be bothered by it. They will have an appreciation for how stressful interviews can be for people new to the industry (which I assume you are since you're interviewing for an internship), and may give you some idea of what it will entail.
Also note that since you're talking to the HR person, they may not even be able to answer the question.
My recommended approach would be to study up on everything that is covered on both the job posting and your resume without asking about it. In my experience, even technical interviews aren't very in-depth, especially in the first round of interviews. They usually understand that even people experienced with a particular technology may not have used it in the particular application used by the company, so some on-the-job learning is to be expected. The interviewers usually just want to confirm a minimum competency.
Since you're in the software field, it also may be beneficial (and fun) to look over questions on CodeReview and some of the challenges over at CodeGolf for the languages you'll be working with. They can both reveal some interesting language constructs (and abuses) if the interviewer wants to throw some curveballs at you.
If you mean asking the interviewer, i.e., the senior developer, it absolutely is inappropriate. The purpose of the interview is to gauge your skills against the job and you asking for the process in advance is like asking about the documents to study and cheat.
If you have a friend, or a recruiter, who has been through the process before, it might be appropriate to ask them but not in a way that, you are just trying to circumvent the process. It is more like "what do they focus on" or "any particular interest of the developer that I should be addressing" are proper things to ask. "What problem he/she ask you to solve, so that I will have a ready made answer to it" is not a good question to ask anyone.
I think the appropriateness depends on the interviewer. But if you want to play it safe, I would avoid asking. Technical interviews usually cover coding and technical questions related to the position and the language or technology you'd be using on the job. If you want some idea of what this looks like, you could try solving some coding tests that are used for job interviews. I can also recommend the book Cracking the Coding Interview, which aims to help with technical interviews and is a very useful read.
Asking the interviewer whether you are expected to prepare anything in advance, should be fine.
The response will probably be a 'no', but it might also reveal what the more exact goal with the interview is.
Personally, I would not study coding riddles as preparation, as that could just as easily increase anxiety on your part.