You could add in:

*Popup "My Computer" "$drives$"

That will open your system's default file manager as if you double clicked My Computer.

Now, if you instead wanted an actual submenu, like in the standard right click menu, you would do:

*Popup "My Computer" !PopupMyComputer

The problem with that is, once it is cached (after first creation), you'll be back at square one with it not updating to show the newly attached drive.

One solution would be to call a !popuprescan before the !bang that opens the menu. It would look something like:

*Popup "Some Caption" !execute [!PopupRescan][!BangToMenu]

That will work, but it will cause a teeny delay before the menu opens. The above assumes xPopup is the popup module.

Lastly, there is probably a better solution if you read over the help file for the latest xPopup (2.x). This is something I don't feel like doing, because it's not very user friendly. :P