I think that if, in good faith, the person is unable to accept more CSAM due to the fact that their hard drive is full, there isn’t an issue. The intent of the law is that, it someone knows something is CSAM, they need to report it. I don’t think the government is going to come hard on Lemmy server owners unwittingly receiving CSAM through federation (though they certainly would want them to report and take down the CSAM on their servers)