To expand on what covecat said: Disks are block devices. The hardware can only read and write whole blocks of data. The block length is typically 512 or 4096 bytes.
What really happens when you use dd to read or write a different number of bytes? Good question. On reading, I suspect that the hardware reads a whole block, and then the dd program only gets the first 440 bytes returned, and the remaining 72 or 3656 bytes coming from the disk are ignored. On writes, I'm pretty sure the action write system call will fail, probably with "invalid argument". If you really want to write 440 bytes, you need to decide what to do with the extra 72 or 3656 bytes. The easiest might be to zero-pad the 440 bytes, and dd even have an option to do that automatically (but doing it would probably require specifying ibs and obs separately). If one wants to keep the last 72 or 3656 bytes on disk unmolested, one would have to perform a read-modify-write operation, as covecat did. This means having to take the desired new 440 bytes, and the remainder from the old sector, stitching them together (as covecat's little script does), then write it back.
