OpenWrt 18.06.2 install on GoFlex Home - Failure

By the way:

1024 = 0x400 = x400 = $400 = 400H

So, to directly answer you - it gave you an error because you didn't count in increments of 0x400. You have been incorrectly counting in increments of 0x1024 (which equals 4132 in decimal - and not evenly divisible by 0x400). Hope this helps. If you don't understand where 0x400 comes from, please read the educational material above.

So, you use a number at least 128k (or in hex 0x20000) larger than the image you're flashing. The blocks are 128k, so do not go over bounds mathematically.