Mac STUCK on disk utility process - “waiting for partitions to activate”

For an Intel Mac, select the physical disk to erase as GUID partition and APFS (top option) assuming you are reinstalling macOS 10.14+, otherwise GUID partition and MacOS Extended (Journaled). I have seen some Macs get confused by drive layout where Disk Utility will show things out of order. When that happens even erasing the physical drive may not be sufficient as it may first require attempting to delete the other partitions, containers, or perhaps the "AppleAPFSMedia" item. It has been a few years since I ran into that issue so I don't recall specifics if that happens. You can try to force quit Disk Utility and start the erase procedure over again, this time on the physical drive. Try to avoid rebooting the iMac since you would likely need to access Internet Recovery Mode instead of local recovery mode.....some people find their Internet/network connection may have issues for this.

It is possible the hard drive on your iMac is failing which could also easily explain the slowness. It may also be why you are needing/wanting to reinstall macOS. I would suggest running the Apple Diagnostics to see if any hardware issues are detected, unfortunately the diagnostics don't usually detect problems....only do this once you must reboot the iMac.

You can also try installing macOS to an external USB3 SSD which may work if the internal drive is bad.

If you are erasing the item listed "Apple HDD....", then you are erasing the physical disk. If this erase procedure is failing, then most likely it indicates the drive is failing.

You can try installing macOS onto an external USB3 SSD drive to see how things work. If the internal drive is failing, then booting from an external USB3 SSD is the easiest fix to keep using this iMac.

Hi,

Resetting NVRAM could solve your issue.

Reset NVRAM on your Mac - Apple Support

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