This thing holds one of those cheap 46x57mm mSATA-to-SATA adapter boards, and has mounting holes in the 2.5" HDD standard form factor.
In other words, you can get a cheap mSATA flash card, a cheap adapter board, and print this in order to make your own cheap 2.5" HDD replacement.
The square and round holes are there to reduce the filament needed and speed printing. If you wanted to be smart, you could replace the big round hole with screw holes for mounting, as the board does have two mounting holes, but they're under the board and require a low-clearance screw heads, so I just ignored them and used a bit of tape -- when the thing is mounted in its caddy, compression keeps it together, so the tape just eases assembly.
A bit more info here.