My 7216 just had the exact same issue...ended up getting a replacement and losing our recorded programs. It would be nice if the external drive could be paired with the account and not a physical DVR in terms of DRM.
If you do replace, make sure to "export series to cloud" from the old DVR first.. the replacement can import the series schedule at least so you don't have to manually enter them again.
Cautious opening is fine. As is various attempts to remove dust, etc that may be screwing things up. You can get the outer cover off fairly easily, but don't try and remove some of the rest as that will easily show as tampering, and could get you blamed for any damage.