@The Sultan:
One common misperception is that FDR's record real time. Before digital they may have, but now the data supplier or the actual recorder buffers the data before writing it to memory. I have heard as long as the last two minutes can be lost.
I don't have data to disprove this, so I suppose it must be considered
possible... but not plausible!
In numerous accident investigations, the last recorded moments of flight data parameters proved to be of great interest and value. Probably, the companies that make DFDRs are well aware of this. Certainly, accident investigation committees (such as the US NTSB) would protest vehemently against a large data loss when an FDR is "knocked out."
I don't have time at the moment to review the FAA CFRs for flight data recorders, but they are pretty thorough and probably include some maximum duration of data permitted to be lost on disconnection. As far as my web search disclosed, actual durations of lost data on disconnection are less than two seconds.
As to "the supplier" of data, in modern fly-by-wire ships like the lost A381, this is an AFDX digital buss operating at 10 megabits / second (minimum). In the case of a disconnect, the last frame may be lost, but this is a matter of milliseconds at most.
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What you heard about loss of as much as two minutes is true ... but NOT about FDRs! Quick-access recorders (QARs), intended for use in airline operations and quality analysis, are NOT designed as an aid to accident investigation. For example, CVRs and FDRs are hardened against violent impact, extreme heat, and submersion ... QARs are not. I found an accident report saying that a QAR lost two minutes of data because of its buffering.