USB Harddrive no longer accessible

Here’s the situation:

I am running Windows XP Pro SP2.

I have a SATA 320GB Western Digital Harddrive in a Vizo Luxon external HDD enclosure, connected to my computer using a USB 2.0 connection.

I have it formatted to show up as my E: drive

It was running just fine until a few days ago.

The Problem:
It no longer shows up in My Computer, preventing access.
It no longer shows up in Disk Management, preventing letter reassignment or reformatting.

The Safely Remove Hardware icon is in the systray, and is even listing the model of harddrive. The drive is also visible in Device Manager under Disk Drives. And of course, it says the Device is working properly…

What is going wrong?