My RAID6 was originally planned with 6 drives, but only had 5 for a while due to space concerns with the case. However, I found as a file, media, and multiple cryptocurrency node it filled up the 2.7TB pretty quickly. So, I got a new case (and some more RAM), which has proper space for 6 3.5″ drives (and 2 5.25″). When migrating to this I decided to add the extra 1TB WD Red NAS drive I bought but have not been able to use.
The case is a Fractal Design Define Mini, and I am thoroughly impressed. Six 3.5″ slots, two 5.25″ external slots, and lots of sound padding on the doors and sides.