If you added the following code to your evox.ini, you could probably get your Z drive back.
First, find where 'Section "Root"' is, and add "Item "Format Z drive",@250" over the first Item, so it looks like this (don't add the 'Launch DVD' line, because it's just showing that you put the 'Format Z drive' line over it):
Item "Format Z drive",@250
Item "Launch DVD",ID_Launch_DVD
Then, find where 'Section Root"' ends, and add this code, below any actions (don't add the beginning braces, because they're just showing where Section Root ends):
Info "Formatting Z:"
Warning "This will create a Z drive!"
Progress "Formatting Z drive..."
Also, some Xbox games will
use the Z partition as a cache to store files, such as maps, so getting rid of it is probably not a very good idea