I've cited above what appears to be a Canadian requirement to pull a cb or otherwise remove power from a CVR after an incident or accident.
The two-hour CVR has been a requirement for several years in America:
§ 121.359 Cockpit voice recorders.
(i) By April 7, 2012, all turbine engine-powered airplanes subject to this section that are manufactured before April 7, 2010, must have a cockpit voice recorder installed that also -
(2) Retains at least the last 2 hours of recorded information using a recorder that meets the standards of TSO-C123a, or later revision; and
(3) Is operated continuously from the use of the checklist before the flight to completion of the final checklist at the end of the flight.
https://www.law.cornell.edu/cfr/text/14/121.359
Canada has proposed upgrading to a similar two-hour standard in future regulations:
Description of the objective
Objective: To enhance the capture of Cockpit Voice Recorder (CVR) information needed for accident investigation purposes.
Description: Proposed amendments would:
increase the duration of CVR recordings to 2 hours;
Recommendation A91-13 - Transportation Safety Board of Canada
I have heard similar claims that the older CVR technology would allow data to be recovered after several overwrites using closely held forensic techniques but I can't find much online about this other than hearsay in forums like this one.
Also, there is some question as to whether the erase function is properly implemented in current flash memory based CVR's. Is the data really gone or can it be recovered as with a camera SD card that has been reformatted but not overwritten?
It is claimed that India's DGCA has mandated removal of the erase feature from units in its aircraft, is this true?