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Within disk utility, the partition icon is grayed out. If I mouse over it, it reads: "Disks with Master Boot Record Partition Maps cannot be partitioned". The history on this drive: It is a 2 TB Seagate external hard drive that the Mac store reformatted for me to use with Time Machine. I had a problem right after I bought my Mac and we did this to backup my machine and literally restore it to a new machine. Since then, I have used it with time machine and now want to re-partition it so I can store backups of additional files to the other partition (once it is created). The current single partition is "Mac OS Extended (Journaled)". Any ideas how to address this issue? I would prefer not to have to erase the disk as part of the process (if possible).

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  • Correction - you cannot partition a drive that is being used with time machine. You could try backing it up, format it, then partition it, then repeat the time machine process. Though I've not done this, so I'm not sure what TimeMachine will do with a partitioned drive. – OzzieSpin Jan 20 '18 at 18:45
  • @Ozzie I've never heard of this restriction, is it something recent? I used to have my Time Machine disk partitioned until about a year ago – grg Jan 20 '18 at 19:13
  • i have 3 partitions on my external drive. one for time machine that works fine. – jmh Jan 20 '18 at 19:22
  • I may be wrong - but last time it reformatted the entire drive. There may have been another reason for this. I'll delete my comment. – OzzieSpin Jan 21 '18 at 11:20
  • I think the take-away here is that a drive currently being used for TM cannot be partitioned. However, an unused drive can be partitioned; after partitioning, each partition will appear as a separate drive, and one can be used for TM. My WD 1TB external has two partitions: one for TM backups, and the other for a less-regularly-updated bootable backup created by ChronoSync. – DavidSupportsMonica Jan 21 '18 at 17:37
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I've just encounter the same problem, where Partition icon is grayed out. I found the solution.

Simply launch this command in terminal and the partition icon will not be grayed out anymore :

diskutil erasedisk hfs+ External GPT /dev/disk2

For clarity, in the above command, the path argument starts from /dev, not "External GPT "

However, this command will erase the disk, make a backup if needed before !

Source : Apple Discussion - Alberto Ravasio

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  • two quick questions: "External GPT " is part of your disk path? Also, if I use this command, the command itself will erase my disk while enabling partition for the future? – TMWP Apr 11 '18 at 16:40
  • @TMWP I really don’t know what External GPT is, but it should work on any disk. And yes, the command itself will erase your disk – AnthoPak Apr 11 '18 at 16:54
  • Thanks! Testing will commence after I am ready to whip out the drive. Not quite there yet, but soon. – TMWP Apr 11 '18 at 16:58
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    Yes!!!! I got a SanDisk 500gb external to back up a newly acquired 2015 MacBook Air (4/128/$500) and could not partition it until I found this magic incantation. Thank you! – marty cohen Nov 7 '18 at 5:37
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    Just FYI: "External" is the new name of the disk, and GPT is GUID Partition Table, the type of partition that you want. You can change "External" to any name that you want. The valid types of partitions are GPT and MBR (Master boot record), but GPT is correct in this case. – Patrick Chu Feb 24 at 16:42
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Another idea: The sample screenshot is only showing 'Volumes', not 'Devices'. In the disk utility, select 'View' and 'Show Devices' to also show the device hierarchy. Then, delete the device which will reset the MBR. (Clicking 'Show Devices' will allow other formatting options thus enabling the partition button)

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TMWP: Yes, that's what I did. Here's the procedure:

  • Go to System Preferences > Time Machine, and turn Time Machine off
  • Make sure that TM disk and your temporary storage disk are both plugged into your computer's USB ports
  • Launch Disk Utility
  • Click on 'Restore'
  • Set the source to your old TM disk
  • Set the destination to your temporary storage disk
  • then start the process

This will copy your TM backups onto your temporary storage disk. Make sure the copy is complete before going further, as the subsequent partitioning of the original TM disk will erase all data on that disk.

Then reformat your original TM disk to create multiple partitions. When the partitioning is complete, you can restore the TM backups from the temporary storage disk onto the new partition where you want TM to reside.

Finally, in System Preferences > Time Machine > Select Disk, select the new partition as TM's target. TM can then be turned back on.

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  • In the disk utility, select 'View' and 'Show Devices'.
  • Select the drive (out-dented entry with the mfgr.'s ID and size) from the side list.
  • Click on the Erase tab in the Disk Utility toolbar. A panel should drop down.
  • In the drop down panel set the partition scheme to GUID. Set the Format type to Mac OS Extended (Journaled.)
  • Click on the Apply button and click on the Done button when it is activated.
  • You can now create partitions.
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