What is Z-drive on a computer?

It doesn't really exist. When computer networking first started, the software used assumed there would be lots of hard-drives attached to the system. Each drive needed a unique way to identify it, and thus they used the alphabet. Floppy disc drives built-in to computers were labelled A & B, and the built-in hard drive was labelled C. Networked drives used the letters D, E, F and so on. Theoretically, you could attach up to 23 hard-drives - although that never actually happened.