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I have a 4TB external Time Machine disk. There are three internal volumes totaling 3.3TB that have been actively backed up to that external drive for a few weeks.

I was troubleshooting something and moving a bunch of files around, so I briefly turned off Time Machine. When I turned it back on, it started backing up... but then it stopped later, saying "there is only 707MB available on the drive, but 3.3GB is required."

It's almost as if it refuses to overwrite the old backups. It wants to start an entirely new backup or something. How can I make it pick up where it left off without having to delete that entire backup and start anew? I don't like deleting backups!

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I'm not sure what the exact percentage is (I think it's somewhere around ~20%). And that percentage of space on a hard disk needs to be kept free in order for the hard disk to perform at best speed. Your hard disk is ~83% full. I'd assume that Time Machine intentionally prompts you with this warning in order to provide best user experience when it comes to backup speeds. So, you should allow it to overwrite old backups. –  gentmatt May 23 '12 at 5:23
    
I can confirm the 20% required disk space for optimal performance. At absolute (!) minimum, it should be 10% though. –  Michiel May 23 '12 at 6:57
    

2 Answers 2

Sadly, the system sometimes does a bad job estimating the space needed.

If you are lucky, you can start adding each drive to the Time Machine Exclusion list and pick one drive with only a few folders to not be excluded (say your boot drive - exclude users).

Also exclude the system files and then ask for a backup. Again, if you are lucky, this lower space estimate will cause the full backup to see that most files are unchanged and then you can slowly remove things from the exclusion list to nurse your drive back to health.

Or, you could take this as an opportunity to make a backup of your Time Machine drive to semi-permanent storage and erase it, start over.

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Gosh darn it Apple. Why do you have to make buggy software –  user15877 May 25 '12 at 5:24
    
I know - this seems the case of good intention gone bad. The overestimate is to ensure you have enough space to write. In your case, the filesystem tracking numbers got redone while TM was off. Again, a nice safety feature that together made you learn about the inner workings when instead you want it to just work. Did you try excluding everything and seeing if you can nurse things back by backing up something that's less than 300M to let the deep traversal happen and lower the free space estimate? –  bmike May 25 '12 at 14:53

Since you state that you have three internal volumes, I am going to assume that when you were:

"troubleshooting something and moving a bunch of files around"

that you moved files between volumes. Time Machine will want to make a backup copy of a duplicate file that is moved/copied to a different volume. It isn't smart enough to notice that it already has a backup copy of the file(s) when they existed on another volume.

It's possible that if you moved a bunch of stuff around while Time Machine was off that it is having trouble reconciling all the changes and it's being compounded by disk drives that are on the full size (as stated before, I agree with the 20% free space threshold.)

Excluding some files from the Time Machine options or using the Time Machine app to delete some old files might help. This is quoted from http://support.apple.com/kb/HT1427

You can also enter the Time Machine restore interface and find files that can be removed from the backup drive itself to conserve space. To do this, select the file(s) and from the Action pop-up menu (gear icon) in the Time Machine Finder window choose "Delete All Backups of...". Be sure to only delete files you are sure you won't need or want to restore later.

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"Time Machine will want to make a backup copy of a duplicate file that is moved/copied to a different volume. " I think it's not even smart enough to track renamed files within the same volume :-( –  Thilo Oct 5 '12 at 8:01

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