Okay, but sooner or later my first back up will get deleted right? What happens if I need files from that first back up? And also can I secure my harddrive with a password?
As long as your backup hard drive is at least as big, or bigger, than the drive you are backing up, you will *never* have any files deleted so that they are completely gone.
The way that Time Machine works is that when the backup drive becomes full (assuming that it is as large, or larger than the drive you are backing up), and you go to backup more recent versions of files that already exist on the backup drive, the only thing that it ever deletes is older versions of those same files. It never deletes the most recent versions of files on your hard drive.
Files on your backup drive that have never changed (or been deleted from the drive you are backing up) are also never deleted.
Users think that as a file ages it becomes more likely that it will be deleted by Time Machine, and that simply isn't true. Only older versions of files for which newer versions have been backed up are candidates for deletion as they age.
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Randy B. Singer
Co-author of The Macintosh Bible (4th, 5th, and 6th editions)
Macintosh OS X Routine Maintenance
OS X Maintenance And Troubleshooting
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