I recently had to transplanted my HDD into another Macbook Pro (after my own broke). I could boot without any problems in the new machine, but for some most (all?) applications wanted me to reenter my passwords (e.g. for all mail accounts, etc.). The passwords were still in the keychain. I could open the Keychain app, unlock the password entries and copy them again into the applications to continue. A couple of days later, I had to repeat the procedure and transplant the HDD again into another Macbook Pro. Again, I had to reenter all the passwords despite them being stored already in the keychain.
So the question is:
Why do I have to reenter passwords that are already stored in the Keychain app after migrating to a new hardware?