If you make another partition out of a partition with data already in it

[levmc]

Could this corrupt the data or would the data go to one of the partitions?
For example, if you had 40gb of data in a 100gb partition, and you split the partition into two 50 gb, would the 50gb of data automatically be forced into one of the partition, or would it be divided into 20 gb in each partition, or could it be corrupted?

 

[Longkeg]

Could this corrupt the data or would the data go to one of the partitions?

Yes and probably yes. I know a “New, Improved” Disk Utility is supposed to be able to create a partition while preserving existing data. I’m sure it works fine but I would never trust it with critical data. It’s just asking for trouble. Back up your data to another drive, do your partition, verify everything is good and reload the data back to where you want it.

 

[joevt]

Disk Utility.app will tell you what it's going to do before it does it so you can cancel. It will say that it will shrink the 100 GB partition (no data will be lost) and create the new partition.

 

[levmc]

Disk Utility.app will tell you what it's going to do before it does it so you can cancel. It will say that it will shrink the 100 GB partition (no data will be lost) and create the new partition.

Does a similar thing happen if you try to do something like that in something similar to disk utility in Windows? (Disk Management?)

 

[joevt]

Does a similar thing happen if you try to do something like that in something similar to disk utility in Windows? (Disk Management?)

Probably. macOS can't shrink NTFS partitions and Windows can't shrink HFS+ partitions.
Also, if you're using Boot Camp on an old Mac that uses legacy BIOS boot to boot Windows instead of UEFI, then you should not do partitioning with Windows, otherwise the hybrid MBR partition table will no longer match the GPT partition table.

 

[Boyd01]

My understanding is that this has all changed with APFS (Apple's new file system) that uses "containers"

"Many files systems, including HFS Plus, support only a single volume per partition. Because free space can’t be shared across partitions, each volume’s size is set when partitioning the storage device, and each volume can only grow into its available free space. In contrast, Apple File System supports multiple volumes within a single partition, which allows all of those volumes to share their free space. All of the volumes in an Apple File System partition can grow and shrink independently; space that’s freed when one volume shrinks can be used when another volume grows."

About Apple File System | Apple Developer Documentation

Use high-level APIs to get the most out of Apple File System.

developer.apple.com

Add, delete, or erase APFS volumes in Disk Utility on Mac

In Disk Utility on Mac, add, delete, or erase APFS volumes.

support.apple.com

 

[joevt]

APFS is another partition type. It is a container that contains one or more APFS volumes. It doesn't make sense to shrink an APFS volume since all APFS volumes share space in the APFS container. You can shrink an APFS partition to allow room for other partitions.