That area full of 1s might not be really erased, though, if power was removed or the device reset during an erase cycle before. Those would be partially-charge

That area full of 1s might not be really erased, though, if power was removed or the device reset during an erase cycle before. Those would be partially-charged cells, and should be erased properly before writing, lest you start getting temperamental read errors later. But repeatedly erasing blocks just to add a byte will wear out the block, as well as take a long time as you point out. I think the solution to these two constraints is filesystem-like book-keeping of which regions are in which states.