Emulab FAQ: Using the Testbed: I've clobbered my disk! Now what?
If you manage to corrupt a disk (or slice), no worries. You can easily repair the damage yourself by reloading a fresh copy of the default disk image. You will of course lose anything you have stored on that disk; it is a good idea to store only data that can be easily recreated, or else store it in your project directory in /proj. Disk reloading is covered in more detail in the Emulab Tutorial.