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I have an external drive with an old mac's time machine backups on it. It takes up about 80% of the hard drive.

What i would like to do is just take the final snapshot that it took of the old mac in order to have at least an archive of it and remove all the incremental backups, so i can use the hard drive for a time machine for my new mac.

How can I go about doing this?

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If you still have the old Mac, the easiest way is probably to wipe the drive and backup once anew. Also, if you migrated your data during installation there might not even be a reason to keep the old backups. –  Daniel Beck Nov 12 '10 at 10:09
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Time Machine stores its backups as plain files and folders. All you need to do is go into Backups.backupdb/yourComputerName/ and you'll be presented with a set of folders named in yyyy-mm-dd-hhmmss format. Simply locate the most recent (or sort them - the most recent will be at the bottom, thanks to its name) and copy it elsewhere. You can then delete the Backups.backupdb folder or wipe the entire drive as you wish.

Alternatively, if you want to copy stuff from your old Mac to your new Mac, you should be given the option to restore from another Time Machine backup when Setup Assistant comes up on the first start up after a clean install (if you've already set up your new Mac, you can also do this after the fact with Migration Assistant in /Application/Utilities/). After it's done that, it'll give you the option of re-using the Time Machine backup as the Time Machine location for your new Mac, so all the history will still be there, and will be gradually deleted once your external disk fills up.

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Have you ever tried this? I gave up after 3 hours of "preparing to delete" — is there a trick? –  Daniel Beck Nov 12 '10 at 10:24
    
It can take a long time to prepare. Time Machine uses hard links so that unchanged files can be included without taking up any more space, and they have to be sorted out before making changes. If it's a large backup, that can take quite some time (if it's any consolation, the actual delete will proceed quite quickly). You may wish to leave it overnight, or copy the backup you want to keep to another drive (at least temporarily) and format the external one completely with Disk Utility. –  Scott Nov 12 '10 at 10:30
    
A note on hard links that I didn't have space for. Essentially a hard link is where you map the same bits on the physical disk to several logical files in the file system. This is what Time Machine does with an unchanged file - it just maps the logical file to the old physical one, so no more space is used up. Deleting takes a long time because you need to check that you do delete physical files that aren't liked to by any logical files, while leaving alone the ones that are still linked to by logical files that weren't deleted. That involves a lot of scanning of the disk, hence the wait. –  Scott Nov 12 '10 at 10:33
    
Thanks for the info regarding preparation time! (Tip: use @Username to notify people if you respond to their comments, so they get notified). –  Daniel Beck Nov 19 '10 at 12:12
    
@Daniel Beck Oh. I didn't realise that actually did anything - I'd assumed it was just a stylistic hold over from Twitter. –  Scott Nov 19 '10 at 12:47
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