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I'm new to the Mac. I am wondering if it's safely guaranteed that I can make a small partition on my active internal hard drive that already contains data and apps, and then delete said partition (later on) so that the drive resorts back to its original size. Without any of the content/data affected. Is this commonplace or are there any safeguards I should put in place (other than backing up the SSD, of course)? Is there a difference based on the file system?

The partition format could be:

Mac OS Extended (Journaled)

Apple File System APFS

MS-DOS or Ex-fat

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    Sure, provided you do this exactly the way Apple would what to you to. Unfortunately, this is closely guarded secret at Apple. Seriously, your question does not contain enough information. How is your Mac currently partitioned? What changes do you want to make? What version of macOS are you using? For example, if you are using APFS, then you might not even need to make a new partition. – David Anderson May 24 '20 at 12:38
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    In any case: backups aren't optional. – Alexander May 25 '20 at 1:14
  • it Will Not Work and you will Lose Your Drive. fantastic external drives now cost almost nothing, do it using an external drive – Fattie May 25 '20 at 1:16
  • Disadvantages of partitioning an SSD? - why would you partition an SSD? Why would you be doing anything with a single HD that you should be doing with separate HDs?... Oh right, because it's a Mac, and a laptop, and it keeps the backup data on the same dive that if fails you have to try to recover from instead of an external backup that's kept powered down and in a safe. – Mazura May 25 '20 at 3:51
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I am wondering if it's safely guaranteed

No. Period.

Nothing is "safely guaranteed." Generally speaking (as at the time of this answer your question is lacking in detail), you can (re)partition without corruption of your data. That's not to say the process is infallible. Problems happen, mistakes can be made. This is why is so important to have a backup of your data.

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    This is not just a theoretical concern! The "new and improved" Disk Utility is rather buggy and crashed on me in the middle of what should have been a straightforward repartition just a couple months ago, leaving my system unbootable (thankfully I had an up-to-date backup). – NobodyNada May 25 '20 at 0:08
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    I agree with @NobodyNada-ReinstateMonica - Disk Partition changes are like a trapeze act. You can do them many, many times, but if you sneeze or get distracted and reach for the wrong bar - you need your net to break your fall. A backup that you know us up to date, you know you have time to restore, and you have actually tested is that safety net. Same with OS upgrades - they don’t break things - it’s when we discover a broken thing now is moved and fails. – bmike May 25 '20 at 16:29
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No - exFAT / FAT / MS-DOS are not embeddable within APFS easily out of the box. Try to not use these if you want easy. Use external disks for all these needs. If you want hard, here is an absolutely brilliant master class on how to set up a Mac for triple boot.

Yes is only if you can keep your data within APFS. You can make many volumes in the overall container and all share free space. This operation has been done hundreds of times on my test macs and I’ve never lost data or been unable to remove one volume and not get the data back or interfere with files in other volumes.

Of course, any system can crash, so keep backups but that has nothing to do with adding more volumes to the APFS container.

Just be sure you don’t partition the disk and you will be fine. Make volumes in the existing container and you are on the easy path. You would want to post details if you go off the easy path, that can cause issues like you may have experienced or heard in the past. Like the comment from David Anderson - partition has very specific meaning and you don’t want that for guaranteed and easy use of Disk Utility to make logical volumes from an APFS managed disk store.

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You can resize your partitions with much more safety than just crossing your fingers and hoping there aren't any files in the section you are giving to the new partition, but you should still back up your hard drive.

Once you have made a backup and unplugged the backup, go to Disk Utility to make the changes. If you give us more details in the question we can give you a more detailed answer.

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  • Any half-baked tool used should move files to be within the boundaries of the smaller resized partition / volume.. But correct - better safe than sorry .. backup your whole drive – eagle275 May 25 '20 at 13:37

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