Xserve RAID Unit - standalone usability

It won't work as a storage unit for accessing/saving data via an ethernet cable. You use the ethernet cable for administering it using RAID Admin. For an XServe RAID to work for its intended purpose such as accessing and storing data it will require a compatible Fibre Channel host bus adapter (HBA) or Fibre Channel switch. You don't need dedicated server hardware such as an XServe or anything else that would be deemed as a server such as a Windows Server. Any Mac or PC with an installed compatible Fibre Channel card will do.

The XServe RAID unit is one of a class of RAID units known as Directly Attached Storage or DAS for short. It is not a Network Attached Storage (NAS) RAID unit.

What Antonio wrote, with more links...

There are a number of Fibre Channel Storage Area Network (FC SAN) devices available from mid to upper-end and enterprise vendors, and some of these options have been tested with OS X Server and the Apple Xsan software. Xserve RAID is an older FC SAN storage array, with older disks.

If you decide to resuscitate this old hardware, then you'll either be running with the box attached directly to the host, or — much more commonly — you'll end up with one or two OS X Server boxes acting as metadata controllers, and Fibre Channel Host Bus Adapters (FC HBA) — and those HBAs can be connected via PCI-X, PCIe or Thunderbolt buses, among other choices — and a fibre channel switch. For an overview of Xsan and such, see page 27 of the OS X Server overview.

If you're looking for more capacity or for speed, then something like the HPE 3PAR arrays or Promise Vtrak E-boxes would be more typical — the Xserve RAID is pretty slow and not very capacious, by present-day standards.

If you're using boxes with Thunderbolt or Thunderbolt 2 I/O and are looking for local direct-connected storage and not a FC SAN investment, then the Promise Pegasus and Pegasus 2 arrays work quite nicely. Or other similar Thunderbolt or USB 3.0 storage, too.

Xserve RAID is what's usually known as Fibre Channel Storage Area Network (FC SAN) storage — while it's possible to directly connect to a host FC HBA without a fibre channel switch, it's much more common to have a switch and multiple hosts all connected and coordinated via Xsan. (There are cheaper options for direct-attached storage.) It's a box of disks connected via a dedicated, storage-private, not-Ethernet-compatible storage network; via Fibre Channel. Metadata controllers and the rest of Xsan is needed just as soon as you have more than one box connected to the FC SAN storage.