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I'm trying to understand what is meant by the wording of this Disk Utility cautionary message that appears when attempting to add a new partition to a disk.

Partitioning this device will delete the data stored on some of the partitions, 
and cannot be undone.

This partition will be added:

"Secondary"

This partition will be erased:

"Secondary"

This partition will be resized:

"Primary"

First point of confusion: It's going to be added and then erased?

Second point of confusion: Disk Utility is supposed to be able to add, resize, and delete partitions without losing data. Sources: 1, 2 So why the ominous message, and who to believe?

Update

I added a new partition and everything appears to have gone smoothly. An answer to another similar question helped me to understand and decide to move forward with partitioning.

This guide, How to Use macOS Sierra Disk Utility to Partition, Erase Drives was also helpful.

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    If what you did solved your issue - please do not "update" your question with a "solved status." Either upvote/accept the post that assisted you or write the answer yourself and mark it as "accepted." – Allan Mar 7 '18 at 15:48
  • @Allan I know, you're right. I just felt like what I wrote doesn't stand very well as an answer, since it's just a few links with little explanation. And the post that assisted me the most was an answer to a different question, which made this an awkward decision. I mostly added the "update" to let others know I had figured it out so they wouldn't waste their time trying to answer a solved problem. I will take a shot at composing a legitimate answer. – Mentalist Mar 7 '18 at 16:11
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Data should not be lost. Just as Solar Mike said, do a backup if you can.

It creates a second partition, but the second will be empty, without any file system, so it will be erased to format it into the file system you choose.

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    I'd backup any important files before you do this, just in case things go bad. – IconDaemon Mar 6 '18 at 16:56
  • Yes just in case you can, but he ask if the data will be erased, not if a problem appears – Chris Mar 6 '18 at 17:16
  • @Chris One of my friends believed the message that the data will be safe , then found it all gone... ALWAYS ALWAYS make an up to date backup before starting ANY partitioning task ... but it’s your data : your choice, just have a search through the site about the number of questions where data lost or drive wiped or partition unreadable occurrs... – Solar Mike Mar 6 '18 at 18:16
  • Right, I never had a problem with my data during partitioning, but I edited my message to had the advice of making a backup. – Chris Mar 6 '18 at 19:51
  • @Chris I never lost my data either when partitioning, but always had a backup... which is partly why I have all my photos and ex-wife lost hers... – Solar Mike Mar 7 '18 at 11:05
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No, most of the time it works just fine. Recently I wanted to partition my 2TB fusion drive into 4. But unfortunately "Disk Utility" was not responding and I had 500gb usable space and rest were gone!

Had to format the drive and then made the partitions and re-installed macOS.

Anything can happen anytime, ALWAYS back up your data first before adding a partition.

  • Thanks. I understand that there is a possibility of something going wrong during the partitioning process. But why does Apple phrase it with certainty: "partitioning this device will delete the data..." as if partitioning was meant to delete data by design, when that is not the case? – Mentalist Mar 8 '18 at 1:21
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For me, my first task would be the backup,

Then I would do the partitioning task - if it goes right no issue you have an up to date backup...

If it goes wrong - you have an up to date backup.

If you don’t do a backup and it goes wrong - your advantage is you have a lot more free space on your drive...

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