Which dashboard are you running?
By default, XBMC4Xbox-based dashboards do not list the cache drives: X, Y and Z when you FTP to your console.
If they are in fact missing/deleted, UnleashX can easily re-create the X, Y and Z drives . Your installation of UnleashX may have a Clear Cache menu option that will create them and clear UnleashX's cache file. If not, edit UnleashX's config.xml file to add the following lines where you want them in the menu:
<List Text="Format cache drives: X, Y and Z" Sort="Off" Batch="True">
<Item Action="Format" Arg1="X"/>
<Item Action="Format" Arg1="Y"/>
<Item Action="Format" Arg1="Z"/>
</List>
Somewhere inside the <List Text="System" Sort="Off" Auto="On"> ... </List> section so it's not in the top-level menu.