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I want to create a second a partition in my hard disk where one (about 900 GB) is used for Time Machine and another (100 GB) is used to store info like a thumb drive.

I don't want any existing data to be lost. I know someone who partitioned his hard drive and it is now spoiled and not repairable. I have important info in this hard drive and this is my only one.

For your info, I am using a retina MacBook Pro with Mavericks and want to repartition a WD My Passport for Mac hard disk.

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Yes you can repartition without losing data. Using Disk Utility, perform a repair on your drive to make sure the drive is free of errors (even better, use Diskwarrior if you have a copy). Then unmount your drive but don't eject it. Select the drive in the left hand pane, then go to the Partition tab. On the Partition Layout section click on the "+" to create a new partition. Optionally you can specify the new partition size. When you're happy hit the Apply button and wait.

If you've absolutely critical data on the drive make sure you have a backup (which you should have anyway) but I've done this many times without an issue.

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    This is great since the original poster has a Mac with either Mountain Lion or Mavericks but it should be noted that live partitioning (which allows for repartitioning without data loss) has only been a feature since 10.5 (Leopard). Users with older OS versions (10.4 and older) would need to backup their data to a different disk before repartitioning.
    – Mr Rabbit
    Jul 22, 2014 at 15:06
  • This doesn't seem to work any more. The Partition button is disabled.
    – minou
    Aug 7, 2022 at 13:20
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The reason that everyone always harps on creating a backup before running any kind of partition command is that if something goes wrong, then generally every thing is gone. I have preformed this kind of operation on PC's many times, generally it has gone well, but the two times over the years the process glitched the partition table was destroyed. Once I was able to purchase a low level sector recovery tool to get files out that way, the other I had backups and just relied on them.

If you don't have any backups, and you preform a partition re-size, and there is a problem, you better hope for either backups, or plan on paying for data recovery.

Good luck.

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When i try to create a partition on external after unmounting by clicking the unmount tab in disk utililty the + symbol is greyed out. What am i missing? Thank you

"Yes you can repartition without losing data. Using Disk Utility, perform a repair on your drive to make sure the drive is free of errors (even better, use Diskwarrior if you have a copy). Then unmount your drive but don't eject it. Select the drive in the left hand pane, then go to the Partition tab. On the Partition Layout section click on the "+" to create a new partition. Optionally you can specify the new partition size. When you're happy hit the Apply button and wait.

If you've absolutely critical data on the drive make sure you have a backup (which you should have anyway) but I've done this many times without an issue."

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Just go to disk management select the drive,which shows the external or internal HDD icon right click it then select the option shrink volume.In the new panel showing put the amount in MB you want the new partition to have.lets see you have 500 GB disk yo want to make a partition of 200 GB.So put the value just in MB. Problem solved.

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