> I think what I'm saying is that calling file I/O "blocking" is also not a useful definition of "block". Because I don't really see the fundamental difference between "we have to wait for main memory to respond" and "we have to wait for disk to respond".

In addition to the point elsewhere made that you're sort of implicitly denying the magnitude of the differences here - the latency differences are on the order of 1000s.

The other way of separating is if the OS (or some kind of software trap handler more generally) has to get involved. A main memory read to a non-faulting address doesn't involve the OS - ie it doesn't ever block. However faulting reads, calls to "disk" IO, and networking IO (ie just I/O in general) involving the OS/monitor/what have you are all potentially blocking operations.