a good little four-disk server with real advantages
By adding a PCI card and an M.2 SSD, to install the OS, we get a good little server with four disks with real advantages:
1: a full OS (here a Debian) with disks that can be read elsewhere if the box fails (and not a proprietary DIY like QNAP or Synology with a smartphone interface and disks that can only be read in a box of the same brand and even then, provided it is the right model and version);
2: a really nice form factor: a real server the size of a shoebox.
You can avoid adding an M.2 SSD by installing the OS on a USB stick plugged into the internal connector, but it's a USB2 and it's slow.
The only real fault is the slow boot time, partly due to the processor but mostly due to the HP bios which is monstrous.