So, this is an old post that I wrote last year, but I just want to point out the non-helpful nature of it. I see a lot of posted replies from the old hands that are incredibly helpful and giving of time, expertise, and energy, and I see a lot that are deeply un-helpful, and don't at all address that question being asked. This is one of the latter.
and doing all the serial communication
Serial communication with what? Not the SD card. That doesn't use serial communication.
It was a typo, I said in the header it was over SPI. That doesn't help, I don't know why you asked that.
at 2.5V (for reasons to do with being very space constrained).
How? Arduinos are typically 5V devices.
Why does that matter? Did you answer the question? Replying to a question with an un-related question isn't helpful. As it happens, I was using a voltage dividing circuit that output 2.5V, but that's actually irrelevant. I see that you've put a lot of a lot of time into answering questions here and I'm sure that a lot of people have greatly appreciated the help and advice that you've given them, but I don't see how any of this was being helpful or informative. I know that there are a lot of posts on this forum that probably grate at the more accomplished posters here and for my part I'll be sure to only post questions with enough background detail and care in them that I avoid provoking ire, but on conversely, non-answers don't do anyone any favors either.