At least, I think it's the smallest. It's the little sibling of the BurgerDisk and like it, it's daisy-chainable, which means you don't have to choose between a hard drive or your floppy drives.
It's available at https://www.colino.net/wordpress/shop/apple-ii/dominodisk/
BurgerDisk is a Smartport hard drive for the Apple II. Its main unique feature is that it is daisy-chainable.
As all Smartport SD-based hard drives, it's more adapted to the non-slotted Apple II, like the //c and IIgs.
Here are a couple pictures of the finished device:
The project's homepage...
Your first question in probably: Why make another Smartport SD hard drive?
The answer is: to finally have a daisy-chainable hard drive, and also free software / open hardware.
BurgerDisk, like the SP2SD, is based on SmartportSD. It fixes two bugs in its firmware (too slow init to be visible...