Sometimes, when you're confident enough to swap the head with one donor and you're expecting the recovery of the lost data but after the swap, you suffer something unexpectedly. There may be a lot of bad sectors, the drives with replaced head are clicking, detected but not readable at all, etc.
What can be the reasons for above head swap failure?
* The platter is contaminated when you open the hdd;
* The clean room is not qualified;
* Improper clean room replacement even if it's a tiny mistake;
* Many people use paper or plastic or eraser spacer strip to separate the heads and this operation undoubtedly lower the success rate of head replacement. After the head swap this way, there will be more or less bad sectors or the heads don't work at all after the swap.