While your tablet is connected to your PC via USB, this is normal. However, while not connected, you should be able to see and access the drives via the file manager. If you are unable to, there may be a problem with the file systems on the flash cards.

As a side-note, most applications can be installed easily through the Application Manager without ever needing to visit a webpage. If you haven't done so already, open the Application Manager, then: Menu > Tools > Application Catalog > Select maemo extras > Edit > Uncheck Disabled.

After doing this, the application manager will automatically refresh (after hitting OK to both dialogs) and you can now see many applications when you Browse installable applications.

Of course to do this you need an internet connection.