Hold the Exit button on the remote until the "power" light blinks on the PVR then press - down, down, 2 in quick succession. (The light blinks on the PVR, not on the remote). This should work on the 8642 (NB2) and the 9865 (NB3), as well as the 4642 STB.
With the 8300 there are 39 pages of diagnostic screens. I don't know how many the Nextboxes have. You just page through them until you see one that refers to the hard drive and see if there are any bad blocks. Most of the screens mean nothing to me.