Difference between alt_write_flash and alt_erase_flash_block/alt_write_flash_block

With NOR flash (which the above sounds like) it is also possible to rewrite an area provided that you are only trying to clear bits.

So you can have a 'partial write' function that will do a read/erase/modify/write if any bits need to be set, and just a write if bits only need to be cleared.

With a caches for a single full sized flash sector and a few 512 ones I got acceptable performance for a FAT16 filesystem.

Although you really need a flash-friendly filesystem that treats both 0xff and 0x00 as 'unused' markers, and preferentially uses the 0xff entries.