Secondary Storage in Von Neumann Achitecture
The von Neumann architecture concerns only memory and CPU, not peripherals or devices.  Storage, such as a hard disk or SSD, is typically a device, and thus is outside the scope of what's normally shown in the von Neumann architecture.  Thus, in a picture of the von Neumann architecture, the block labelled "memory" should be understood to refer to RAM -- not hard disk or SSD.

In particular, in the von Neumann architecture, the CPU can access memory through a bus (the memory bus).  In real computers, typically the memory bus cannot be used to read or write the hard drive; there's a separate path for accessing devices like the hard drive.  (That's from the perspective of the architecture/hardware level.  However, the lines can admittedly start to get blurred a bit from the user application's perspective, given memory-mapped devices, virtual memory, and demand paging.)