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.
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Just to clarify, do you notice the clicking sound when completing a specific function (e.g changing a channel, pressing record button, navigating) or is it constantly there? Also, did you just recently start experiencing this or has this persisted for a while?