> But in what sense are the flash drives base 2?

In the sense that the memory chips inside are base 2 capacity and the sole reason you are seeing less is because the flash control chip just reserves some space.

What's the point of pretending you don't understand someone's comment and asking for clarification?

Fundamentally, address decoders are made to operate with binary inputs. That constrains the scope of what they address to powers of 2. With large capacity storage that becomes less relevant as they are overprovisioned with some redundant capacity, but in reality such devices are implementing Nx2^M storage, not Nx10^M.

Compact Flash, SD, thumb drives are all advertised base 2. SSD are advertised usually base 10 but I've seen some advertised base 2. No consistency. But that's marketing for you.

The number of memory cells will be as they need to be addressable...