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HT1427: Mac Basics: Time Machine backs up your Mac

Learn about Mac Basics: Time Machine backs up your Mac

HT1427 How much drive space should i use for time machine osX

233 Views 3 Replies Latest reply: Apr 3, 2012 8:57 PM by martinjc RSS
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Apr 3, 2012 8:47 PM

If I alocate 1tb of my 3tb external drive to back up my 1tb internal hard drive, will the external eventually fill up even if my internal is only half used?

iMac, Mac OS X (10.6.8)
  • Niel Level 10 Level 10 (234,655 points)

    Yes. Time Machine will eventually take up all the space allocated to it.



  • Kappy Level 10 Level 10 (221,025 points)

    A Time Machine volume should be at least twice as large as the capacity of the volume it backs up. The nature of Time Machine is that eventually the backup drive will become full. See:


    About TM "Backup Drive is Full"


    Alert TM only deletes older files if they have been deleted from the source and when TM needs space on the backup drive for a new incremental backup. Time Machine "thins" it's backups; hourly backups over 24 hours old, except the first of the day; those "daily" backups over 30 days old, except the first of the week. The weeklies are kept as long as there's room.


    So, how long a backup file remains depends on how long it was on your Mac before being deleted, assuming you do at least one backup per day. If it was there for at least 24 hours, it will be kept for at least a month. If it was there for at least a week, it will be kept as long as there's room.


    Note, that on a Time Capsule the sparsebundle grows in size as needed, but doesn't shrink. Thus, from the user's view of the TC it appears that no space has been freed, although there may be space in the sparsebundle.


    Once TM has found it cannot free up enough space for a new backup it reports the disk is full. You can either erase the backup drive and start your backups anew or replace the drive with a larger drive.


    For much more about Time Machine visit


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