Aterpster
The CVR in the Hoot Gibson case was erased due to a slow transfer to external power, something that was known about and apparently the NTSB chose to ignore.
In any case, the aircraft was not shut down for a longish period after landing, the bit of the tape the investigators wanted was overwritten during this time.
It was not possible to erase the recorder deliberately, the damage to the undercarriage meant that the squat switches would not close and so the erase mechanism would not work. The NTSB also chose to ignore this, accusing the crew of deliberately erasing the evidence.
There's a good discussion of this in Stan Stewart's book "Emergency - Crisis in the Cockpit".
A quite disgraceful sequence of events in my opinion, the investigators should be ashamed of themselves.