If you do in fact mean a "hard drive", there are apps such as FileMagnet and Air Sharing that let you store files and view them on the fly, but each one requires you to use their software on your computer to add the files to the Touch. Unfortunately there's no Disk Mode on the Touch the way there is on the Classic, so you don't have the option of getting the Touch to show up in the Finder as a removable drive 🙁