I know that there's a "Change Disk..." button in Time Machine, but what if you want to migrate the data (hours, weeks, and maybe even months) to a new, larger disk without starting over again?


6 Answers 6


I recently had to migrate all my TM backups to a larger drive.

You can successfully complete the operation using to the following tutorials:


SuperDuper! will do the job. You can use it for free to do what you want, and pay only if you want to continue to use it to do efficient incremental backups using its "Smart Update" feature.


For 10.6, Apple has included the following in Mac 101: Time Machine:

Mac OS X v10.6: How to transfer your back ups from your current hard drive to a new hard drive

If you upgrade to a new hard drive with a larger capacity, you may want to transfer your existing Time Machine backups to it before using it for regular backups. To keep your existing backups on your new, larger backup hard drive, follow these steps:

  1. Connect your new hard drive.
  2. In Disk Utility, make sure it has a GUID partition and is formatted as Mac OS Extended (Journaled). If needed, reformat the drive as Mac OS Extended with a GUID partition. If you do not know how, see the steps in this article. Note: Back up any data on the hard drive before you format it.
  3. Select the new drive's icon on the desktop and choose Get Info from the File menu.
  4. Make sure "Ignore ownership on this volume" is not enabled.
  5. Open Time Machine preferences in System Preferences.
  6. Slide the Time Machine switch to Off.
  7. In Finder, double-click the current backup hard drive to open its Finder window.
  8. Drag the folder "Backups.backupd" to the new hard drive.
  9. Enter an administrator user name and password, then click OK to start the copying process. This may take some time to complete because all your backups will be copied.
  10. In Time Machine System Preferences click "Select Disk…"
  11. Select your new hard drive, then click "Use for Backup."

Now, on your new hard drive, you will have all of your existing backups from the previous backup hard drive plus have more room for more new backups.

(There are also instructions for Time Capsule and Mac OS X v10.6: How to transfer your back up from an existing Time Capsule to a new one.)

As a side note: when you are using a sparse bundle (like for remote backups) and if you made backups of really large files, then deleting those files from the backup (either manually or when Time Machine removes expired daily and hourly backups) will not automatically reclaim the disk space -- until Time Machine really needs it.

You can reclaim the space manually, and it may free a lot of space (possibly making migrating to a larger disk unnecessary, or at least giving your new disk all the possible space). See What is Time Machine doing? on Server Fault.


This won't be a help to you right now, but for someone starting from scratch who wants a portable TimeMachine backup (with the side benefit of being able to limit the portion of the disc used for TimeMachine), I suggest using a mounted disk image (.dmg) as the target for TimeMachine's backup disk.

So as an example, let's say you have a 2TB external drive, but you want to reserve 50GB for the TimeMachine backup and you'd like to be able to move the backup to another external drive, even one which already has content. You could do this:

% cd /Volumes/<extdrive>/
% hdiutil create TimeMachineImage -size 50g -fs HFS+ -volname TimeMachineDisk
% open TimeMachineImage.dmg

You would then go into the preferences and choose /Volumes/TimeMachineDisk as the backup drive. You can be assured that you won't grow over the 50GB size you want to use and the space is already reserved on your external drive, so it won't grow larger to squeeze you out of space.

You could then move TimeMachineImage.dmg to another drive for TimeMachine backups. Or simply for offsite protection.

This also opens up some other options, the most important of which is the ability to encrypt your TimeMachine backups. To do this, you simply enable encryption for the dmg, so you'd use something like:

% hdiutil create TimeMachineImage -encrypted -size 50g -fs HFS+ -volname TimeMachineDisk

The final piece of this puzzle is that you would like to automount the dmg when your external drive is mounted. I'd guess it is possible to do this with Applescript so it happens automatically, but I'm not familiar enough with Applescript to help here. You can create a cronjob, though, that runs a script to mount it:

if [ -d /Volumes/<extdrive> ] ; then
  open /Volumes/extdrive/TimeMachineImage.dmg

And install a cronjob that looks like:

*/5 * * * * /usr/local/bin/<scriptname>

You can also create a sparse image so that it does not suck up the entire 50GB when you first create it by including the -type SPARSE arg in the hdiutil call. The danger here is that you may run out of space on your external drive.

  • As for You can be assured that you won't grow over the 50GB size you want to use and the space is already reserved on your external drive, so it won't grow larger to squeeze you out of space. -- a note from Mac OS X Hints: As of 10.6.3, Time Machine tries to increase the size of the sparse bundle to the maximum available [..]
    – Arjan
    Apr 3, 2010 at 16:17
  1. Connect both drives to your Mac.
  2. Open Disk Utility (/Applications/Utilities/Disk Utility.app).
  3. Click the Restore tab. (First Aid | Erase | Partition | RAID | Restore)
  4. Drag your old drive into the Source field.
  5. Drag your new drive into the Destination field.
  6. You may want to check Erase Destination.
  7. You can be make the computer ensure that the transfer worked by unchecking the Skip Checksum checkbox.
  8. When the transfer is complete, unmount and unplug your old drive. Burn it as an offering to the gods of hard drives. (Optional, but highly recommended.)
  • 1
    Those hard drive gods must constantly be appeased, lest they wreak swaths of bad sectors on my new disk.
    – Kevin L.
    Jul 16, 2009 at 6:37

I think you can just copy over all of the contents (including hidden dot-files). It may be safer to use a block-level copy, with something like Carbon Copy Cloner.

  • But will this work for a larger partition (i.e. my current predicament: migrating an overburdened 320GB Time Machine disk to a 1TB new disk)?
    – Kevin L.
    Jul 15, 2009 at 8:38
  • I believe so. If not, well then you've still got your files and both disks; you can try something else. You should consider going to an Apple store and asking a Genius or other rep.
    – jtbandes
    Jul 15, 2009 at 8:52
  • If your backup disk uses a sparse bundle (like for a remote backup) then file copying is indeed possible. However, when not using a sparse bundle then file copying will explode the disk space usage, as hard links are then copied as normal files. See code.google.com/p/timedog/wiki/UsingTimecopy
    – Arjan
    Jul 18, 2009 at 16:22
  • Does a block-level copy avoid that problem?
    – jtbandes
    Jul 18, 2009 at 16:49
  • Yes, a block-level copy preserves hard links. Now, if your file system is somehow giving problems (which is NOT the case if one just wants to duplicate the disk) then the link I gave to code.google.com/p/timedog/wiki/UsingTimecopy might be helpful.
    – Arjan
    Jul 18, 2009 at 17:24

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