I have a colleague who takes photos of the board after every session. Every one, without exception, even when the session was fifteen minutes of scratch work that led nowhere. I understand the impulse. You don't want to lose the thinking.
I used to do the same thing. I have a camera roll full of blurry whiteboard photos from four years ago. I've looked at maybe three of them since taking them.
The fear of erasing is really a fear that the thinking was valuable and might be needed again. That fear is mostly wrong. The thinking that was valuable changed you — it's not on the board, it's in how you now understand the problem. The whiteboard captured the artifact of the thinking, not the thinking itself.
When I erase something, I try to do it with some intentionality. I'm not deleting it in frustration. I'm declaring that this chapter is closed. The problem is resolved or abandoned or transformed into something different. The clean surface is not emptiness — it's readiness.
There's a version of this I've noticed in my own thinking patterns. I hold onto half-formed ideas on the board for too long sometimes, not because I'm actively working on them but because erasing them feels like admitting they didn't work out. That's backwards. A thing that didn't work out is worth erasing quickly, so you can see what's actually in front of you.
The photos I take now are of the things I want to keep — the final version of a design before we implement it, the structure of a talk I'm preparing. I don't photograph the working versions. The working versions are meant to be erased.