A few years ago I linked my account to my friends PlayStation account, and now I have my own PlayStation. He doesn’t know his old email and password, how can I disconnect it if I don’t have the email or password?
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Your friend will need to use their platform's account recovery options in order to recover their login information, otherwise that Cross-Save will remain connected and your account linked. It's unfortunate that you chose to use Cross-Save in a manner that it wasn't intended for.