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I am using Time Machine with an external drive connected to a Macbook running Mountain Lion. I use backup encryption.

Whenever I connect the external drive TM asks for my password. The password I'm sure I used doesn't work, nor any others I've used in the past. However, it still backs up when I choose 'Backup now' and I appear to be able to use Time Machine to view old files in other backups!

Any ideas what may be causing this?

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This seems to be a mix-up with the Key Chain. As soon as I enabled adding it to keychain it accepted my password and stopped asking to re-enter it. On removing the recently added item in the key chain it started asking again. It seems Time Machine can get into a state where it both recognises you have a valid keychain, but also asks for a password. – Tony Sep 2 '12 at 9:58
It's possible you had multiple conflicting keychain entries (e.g. one in your personal keychain and another in the system keychain), and one of them was the wrong password... – Gordon Davisson Sep 2 '12 at 15:03

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