Can't install older OS on partition

Cartoonguy Said:

"It is mounted and accessible and reads and writes, but I can't get the OS to install on it."

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Installing This:

• From a USB installer

Go Here: How to create a bootable installer for macOS - Apple Support.

• Use Option-Command-R

How to reinstall macOS from macOS Recovery - being a new partition, as the site reads: If macOS Sierra 10.12.4 or later has never been installed on this Mac, Option-Command-R installs the macOS that came with your Mac, or the closest version still available. And Shift-Option-Command-R isn't available.

Cartoonguy Said:

“Can't install older OS on partition: [...]The clue is that it says it will take 16.5MBs of space, no mac OS can only be 16.5MBs. The Install macOS Sierra.app is 5.05GBs[...]”

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Thank you for the clarifications.

Please Report this File’s Bug to Apple:

Having troubleshot all of this, the last step here, so it seems, is a matter of feedback. I would report this bug in the update to Apple. Apple should definitely be made aware of an update reporting itself as about 3% of its actual size..

1. Go Here: Feedback - macOS - Apple
2. Select: “Software/Hardware Compatibility” for the “Feedback Type”
3. Select: “Software/Firmware Update” for the “Feedback Area”
4. Proceed from there as necessary

Cartoonguy Said:

"Can't install older OS on partition: I have a partition and I am trying to install Sierra on it because I need to run software which needs Sierra. When I run the installer, it skips past the destination option and says it will install on Macintosh HD,[...]"

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Mount the Partition:

You've just created a partition for this, correct? If that is the case, mount it, doing so in Disk Utility, while booted in Recovery Mode. Having just been mounted, see if it now gives you the option to use it. If not, then while in Recovery Mode, run First Aid, and see if there is anything that needs to be fixed.

Mounting it Anew:

As a last resort, just delete this newly-created portion, and then create a new one, mounting afterwards, and then proceeding with the install.

Cartoonguy Said:

“Excellent! Thanks. It worked to install the original OS and then go from there. Didn't even know you could do that.”

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You are welcome! Glad it worked out!

You can lean something everyday on these forums. And I’m proud to take part.

Oh well what you missed was that the Installation window you get does not install macOS Sierra but converts the InstallOS.pkg to the Install macOS Sierra.app which you find is created in the Applications folder. The clue is that it says it will take 16.5MBs of space, no mac OS can only be 16.5MBs. The Install macOS Sierra.app is 5.05GBs in size and you double-click on that to start the installation of macOS Sierra, follow the screen prompts and you will see one that asks you which Disk/ partition you want to install macOS Sierra. It's that simple.