Keychain error -25308 while trying to add drive to Time Machine
I was having this same problem. According to discussion at this thread on Ars Technica, it’s caused by a bug in iStat Server. Are you running this?

I was, and can confirm that the work-around solution posted there of removing iStat Server and deleting the iStat Server entry in /Library/Preferences/com.apple.security.plist has worked for me.  The relevant entry looks like this:

&lt;key&gt;DefaultKeychain&lt;/key&gt;
&lt;array&gt;
   &lt;dict&gt;
      &lt;key&gt;DbName&lt;/key&gt;
      &lt;string&gt;/Library/Application Support/iStat Server/iStatServer.keychain&lt;/string&gt;
      &lt;key&gt;GUID&lt;/key&gt;
      &lt;string&gt;{87191ca3-0fc9-11d4-849a-000502b52122}&lt;/string&gt;
      &lt;key&gt;SubserviceType&lt;/key&gt;
      &lt;integer&gt;6&lt;/integer&gt;
   &lt;/dict&gt;
&lt;/array&gt;


Bjango, the developer of iStat Server, has further posted on the Ars Technica thread that a new version of iStat Server is now available.  I have not independently verified that this addresses the problem, since the above work-around had already taken care of my issue, but their release notes say simply “Fixed a Keychain issue.”  Both the new version and the release notes can easily be found at the iStat Server link above.