According to the box from a compatable connector from Radio Shack, it is a Coax-Style DC power plug/socket 4.0mm O.D., 1.7mm I.D..
Many different devices uses the same connector, and often the same voltage. Most notable is the Sony PSP which uses the same connector and voltage, so external batteries, USB power adapters and car and wall adapters designed for PSP work with Zauruses.
I don't know of any harddisk enclosures that use the same plug, though I haven't looked. However, I do have a couple of enclosures that use an additional USB plug for power. You could either get a USB charge cable for the Z and then a USB wall charger to power both, or rig up an adapter with parts from Radio Shack to make it USB (Coax socket, then a USB socket).