A keychain file that leaves one Mac behind will not give up its contents on the next one, and the lockout has nothing to do with a mistyped password. Copying the file still works. Opening it does not.
The behavior was documented on September 8 by Rich Trouton at Der Flounder, then confirmed three days later by Howard Oakley at the Eclectic Light Company, who ran checks of his own. Both trace it to the Secure Enclave, which now contributes a secret of its own whenever a login keychain is unlocked. Present that file to a machine unable to produce the same secret and admission is refused, with no route back.
Oakley’s virtual-machine trials stretched the finding further. A guest running Tahoe 26.6.2 could not open a keychain taken from a physical M4 Pro Mac mini, and two guests trading the same file between themselves fared no better, even though virtual hardware of that kind carries no enclave at all.
The supported path still behaves. Where a user is handed from one Mac to the next, or from a Tahoe system into a Golden Gate 27 release candidate, Migration Assistant reproduced passwords, certificates and the remaining keychain contents exactly as expected.
Put plainly, a copied keychain is no longer a spare set of keys. A thief who lifts the file inherits nothing usable without the machine that produced it, and that is a quiet gain for anyone whose laptop walks away. For the honest job of replacing hardware, Migration Assistant covers it, and the manual file copy deserves retirement.
