High Sierra 10.13.5 - delete original partition

Backup up everything to a Trusted Backup, in case something goes wrong. Make sure the files are on that Backup in good order.

Now that you have that emergency backup on hand, which you needed to create anyway, it is far, far less trouble-prone to just erase the entire drive and set it up as you like, and restore from your Trusted Backup.

To erase the drive completely, you must not be running anything from it. That means you can not be running "regular" Recovery, because it comes from a partition on that drive.

>> Booted to a USB stick Installer should be fine.

>> Booted to Internet Recovery should be fine, because it puts everything it needs in dozen or more RAM disks so as to leave the drive free.

Disk Utility:

MacOS tires to protect you from deleting Drives with data still on them. Instead of being honest about it, it simply refuses, and provides no helpful reasons as to why it failed.

You need to be absolutely certain you are deleting the right items. Then ask it again. On the second time in a row, it will often comply.

In my opinion, this is a stupid way to do things. MacOS should always provide full information so that the User can make an informed decision. The way this works/does not work now is "not the Apple way" in my opinion.

If you wish, while you still have a fully functional MacOS available to you, you may want to make a Bootable USB-stick Installer and work from that device. It will contain all the Utilities as well as the Installer image needed.

Or you can use Internet Recovery, and download all from the Internet. Temp files will be stored in a dozen or more RAM disks, leaving the usual boot drive free to be modified.

How to create a bootable installer for macOS - Apple Support

If you erase and name your USB stick initially "MyVolume" you can cut and paste the Terminal commands from the article for direct use. Be sure to select the entire contents of the scrolling box.

Whatever method works for you, be sure NOT to use regular Recovery, because that Recovery partition is on the drive, which precludes making the changes you need.

About macOS Recovery - Apple Support

Make sure that Disk Utility is showing you Device-View, not the completely useless Volume-View. Select the drive by Hardware-name and choose ERASE. You want the default GUID partition Map and a MacOS HFS+ Extended (journaled) Volume. The Installer will convert that to Apple File System (APFS) during the Install.

Then Re-Install MacOS.

How to reinstall macOS - Apple Support

At first run, it brings up Setup Assistant, and invites you to create a new Userid, but DON'T. Instead, tell it to use the information on your backup drive, and it will give upon a screen like this:

Set up your new Mac - Apple Support

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