Perhaps the FBI had a valid reason for not doing NAND mirror [0]. However, I maintain that it is incompetent for the FBI director to testify before congress on the case and not be prepared with a basic answer to why the most obvious technical answer would not work. Similarly, with him saying in a press conference that it would not work.

Of course, the FBI director does not need to know this level of detail about every case, but given how much he appears to have known, he should not have been speaking about the case beyond directing people to ask the person in charge of the case.

[0] Although, I would argue, in cases where no alternative exists, if the FBI policies prevent NAND mirroring, they should be revisited. Even if it has the potential to be destructive, they would only be destroying otherwise unusable information.