It sort of depends, but in general flash cannot write a page without erasing it first. Sometimes you can set, but not un-set bits (can be a 0 or 1 depending, us

It sort of depends, but in general flash cannot write a page without erasing it first. Sometimes you can set, but not un-set bits (can be a 0 or 1 depending, usually 0 is 'set') without erasing a page. At the lowest level this means to update a page you have to first read it all back out, apply your modifications, then write back in a single operation. You must always write the full page size. In high level contexts like SD cards or SSDs being driven by an operating system, all of this is handled transparently in the background. If you're using raw flash in an embedded context, you usually have to manage this yourself (or let a library do it for you).