Safari replaced all my bookmarks with those from another of my computer... without asking. How do I keep this from happening again and how do I restore the old bookmarks?

1. Open the iCloud preference pane and uncheck the box marked Safari.

2. You can only restore the whole bookmarks file to a previous state. It's not possible to restore individual bookmarks.

Triple-click anywhere in the line below on this page to select it, then copy it to the Clipboard by pressing the key combination command-C:

~/Library/Safari

Quit Safari.

Select

Go ▹ Go to Folder

from the Finder menu bar. Paste into the text box that opens by pressing command-V, then press return.

A folder window should open. Restore the file named "Bookmarks.plist" in that folder from a backup that predates the unwanted changes.

With the Safari folder open as indicated above, enter Time Machine and select a snapshot from which to restore.

If you only have two snapshots in Time Machine, you need a larger backup volume. But that's not going to help you restore the bookmarks.

Back up your iOS devices. Turn off iCloud synchronization of Safari on all devices except one of those. Quit Safari and delete the bookmarks file on the Mac, then enable Safari sync on it. Launch Safari. The bookmarks should sync from the iOS device.