What have you tried doing?
Go to control panel double click system, go to the hardware tab, choose device manager and then under "Disk Drives" find the drive and right click it. Choose "Uninstall"
Go to my computer right click and say "Disconnect" to your Z drive
Turn the PC off, cut the power to the HDD, turn the power back on and reboot.
Upon reboot it should reconize the HDD and reinstall it.