Storage on the 3DS' SD card is calculated in the 3DS eShop using strange units called 'blocks'. While these may have been an attempt to simplify things for the average user, it also makes it much harder to do things like estimate the download time for a piece of software or how many demos I could fit onto an SD card. So for these purposes I wish to find out, how big exactly is a 'block', in terms of actual bytes?
According to various google results 1 block is 128kb.
Using blocks to describe volume capacity isn't very uncommon. Blocks on hard drives are usually known as sectors. This isn't transparent and an end-user does not need to know about this, especially as the physical layer on the harddrive is further abstracted by the overlaying filesystem with its own restrictions on allocation sizes.
I assume that Nintendo uses blocks to describe minimal allocation sizes, so a file that is smaller than 1 full block will still use one full block of space as there is no means of allocating let alone accessing partial blocks.
[A] block [...] is a sequence of bytes or bits, having a fixed length (a block size). [...] Blocked data is normally read a whole block at a time.