Earlier quoted context omitted.
3U and 4U x86 whitebox servers designed for any standard 12"x13" motherboard, where the entire front panel was hotswap 3.5" HDD bays were already a thing many, many years before backblaze existed. What wasn't really a thing was servers with hotswap HDD trays on both ends (like the supermicros) and things that were designed with vertical hard drives dropped down from a top-opening lid to achieve even higher density.
That's not the backblaze design. The backblaze design is that the drives are individually hot-swappable without a tray. 60 commodity SATA drives that can be removed and serviced individually while a 4U server continues to operate normally is pretty amazing.
> things that were designed with vertical hard drives dropped down from a top-opening lid to achieve even higher density.
the backblaze storage pod system and some 3rd party derived designs have the SATA/power ports on PCBs facing upwards, mounted at bottom interior of rack chassis, and 3.5" HDDs mounted vertically down into the system.