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Can anyone verify that this can be done before I do it? The Gui let me add a new partition and I could resize the old one downward to give space to the new one. It stopped at a certain point above the actual amount of data in the partition, so I'm guessing that it will not let you destroy existing data. Which is great.

But it would sure be nice to know for sure. Thanks!

KenNickels wrote:Can anyone verify that this can be done before I do it? The Gui let me add a new partition and I could resize the old one downward to give space to the new one. It stopped at a certain point above the actual amount of data in the partition, so I'm guessing that it will not let you destroy existing data. Which is great.

But it would sure be nice to know for sure. Thanks!

Disk Utility will absolutely let you erase data. It does give you warnings before you proceed. So, while it's hard to do accidentally, it's certainly not impossible.

Doing so without having a current backup is never recommended

Thanks, Mike, I did make a backup. But I hope I don't have to use it. You didn't say ... have you ever successfully shrunk a partition? What I see on Apple and elsewhere is the word "resize" but never shrink. And all of the examples are for expanding a partition. I wish I had a spare disk to test, but I don't. Thanks.

When you add partitions, you are shrinking others.
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Yes.

mikehalloran wrote:When you add partitions, you are shrinking others.
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Yes.

In Mac OS it's a little unclear, that's all. Windows has a "shrink partition" function, which is more straight forward, especially if that is your intended goal. In Sierra, you must add a new partition, and in the sizing of the partition, it necessarily borrows from the adjacent partition, thereby shrinking it. So it is rather a by-product of adding a new partition. However convoluted, if you understand this, it is safe and predictable and will NOT let you overwrite any data in the adjacent partition, which is what I was worried about. So, mission accomplished.