Terastation WS5420DN

Specs

CPU	Dual Core Intel(R) Atom(TM) CPU C3338 @ 1.50GHz
Memory	8 GB DDR4 SoDimm(Upgradable?)
Bays	4
Network	2x 10/100/1000 Mbps, 1x 10Gbe Aquantia Corp. AQC107
Sata	4x SATA III
USB 2.0	No
USB 3.0	2x Back uPD720201
Serial port	No
VGA	Yes
Fan	1x 92mm
Power	Internal AC 100-240V, universal 50/60Hz

Stock Firmware Installation

The device comes with Windows Storage Server installed and can be recovered via the included SD card.

Custom Firmware/Operating System installation

These devices run on an Intel Atom processor with a fairly typical BIOS and include a VGA out and support USB keyboards, you should be able to run virtually any operating system that will run on a normal PC depending on drivers/etc.

For tools to help install and configure Debian on these devices, see Installing Debian on Intel Terastations