I replaced my old QNAP TS-419P2 (noisy 4-bay server with slow ARM cpu, too less memory, etc) with a custom built server running GNU/Linux Debian (stretch).
It mainly serves as file storage (smb), but is also my test environment for several projects I work on with these services enabled:
custom debian repository
The mainboard supports up to 4 SATA devices of which one (SSD) contains the OS and two form a software RAID level 1 (mirroring), resulting in 4TB net storage space. The main reason for only having 4 SATA ports is that I wanted the server to be as silent as possible; the mainboard with the integrated CPU has passive cooling, the case fans do enough airflow to keep the CPU, RAM and storage devices cool. Additionally there is an old external USB-connected harddrive with 500 GB of backup space.
This project is running stable, but (as always) stuff is eating up storage space. So I am currently looking for another mainboard/CPU combination with low TDP and support for at least 6 SATA devices.
Donations are very welcome, especially if you like me, the project or just are generous: https://www.amazon.de/hz/wishlist/ls/VWN8BRYCE3YZ?&sort=default