I had 16 hours and 35 minutes of HD content recorded on my DVR, along with 16 hours and 25 minutes of SD content, and my 8742HDC reported it being 76%. I purged the cache but that knocked it down only to 72%.
In this thread here: http://www.avsforum.com/t/1219141/cisco-8640hdc-hd-dvr-thread/330
there are references to how to run the hard disk drive diagnostics tool, which is kind of like a ChkDsk type utility that attempts to diagnose hard drive problems and repair them, but this procedure (holding down select and volume up buttons while unplugging and then plugging in the box) doesn't seem to do anything on the 8742.
I'm looking for some suggestions. Time Warner Cable's response, from both technical support people I talked to, along with their supervisor, has been to try to tell me this really isn't a problem and that it's totally normal for it to be that full because high-def takes up a lot of space, but I know from experience that that is wrong.