When upgrading to OS X Yosemite on my iMac I took your advice and installed it on an external drive. I like it so far, but even though I’ve switched on and verified Keychain in the iCloud system preference, Safari doesn’t know about my previously saved passwords. When I launch Safari, open its preferences and click on the Passwords tab, the area below is empty. Is there some file I need to copy from Mavericks to make this work?
You needn’t do that. Rather, you just have to give iCloud Keychain a not-so-subtle nudge. Apple outlines the steps for doing so in its Get Help Using iCloud Keychain document.
What Apple suggests is that you switch off iCloud Keychain on all devices that use it. Then, figure out which of your devices has the most up-to-date list of your passwords and switch on Keychain for that device. And finally, turn on iCloud Keychain for all your other devices. This should cause Safari to produce the passwords you’re looking for.
As someone who’s dealt with this issue (and one who also has more than a handful of devices that use iCloud Keychain) I might suggest that you first try a shorter route. And that would be to switch off Keychain on your Yosemite drive, restart your Mac from the Mavericks drive, switch Keychain off and on, switch back to your Yosemite drive and enabled Keychain there.
If you’re successful (as I have been) you’ve saved I-don’t-know-how-many trips to Settings/System Preferences on your other devices and countless keystrokes as you reenter your password and verify iCloud Keychain on another device.