Apple, dropping the article before a product name years ago:
Xserve may exhibit a symptom where USB devices, FireWire devices, and the optical drive do not respond and external mass storage devices do not mount.
Xserve may be locked. Unlock Xserve using the Allen key that came with the server. Once the external mass storage volumes mount, you can lock the server again.
You can determine that the server is locked if the yellow security LED on the front panel is on:
