You have the ability to install your OS directly from Apple via Internet Recovery which if the system was running El Capitan or newer will have the needed firmware update installed to allow this. All you need is directly access to the Internet. In addition your system should have a recovery partition setup during the OS install or update on your current drive. As long as the drive is functional you have that as well.
Here’s how! [https://support.apple.com/en-us/HT201255|Mac startup key combinations] & [https://support.apple.com/en-us/HT204904|How to reinstall macOS from macOS Recovery]
'''Now the fly in the ointment!'''
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Apple allowed the newer macOS’s certificates expire prematurely! Here’s more on this [https://appleinsider.com/articles/19/10/24/if-youve-got-an-old-macos-install-image-it-will-probably-stop-working-today|If you've got an old macOS install image, it will probably stop working today]
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Apple allowed the newer macOS’s certificates to expire prematurely! Here’s more on this [https://appleinsider.com/articles/19/10/24/if-youve-got-an-old-macos-install-image-it-will-probably-stop-working-today|If you've got an old macOS install image, it will probably stop working today]
I do recommend sticking with macOS Sierra as the best for your system. The issue is the limits of SATA drives. Apple upgraded the file system to APFS from the older HFS+ which runs better with SATA drives.
Here’s the updated installer [https://support.apple.com/en-us/HT208202|How to upgrade to macOS Sierra] jump down to Step 4 for the link.
To setup a bootable OS installer thumb drive follow this guide: [https://www.macworld.com/article/3092900/how-to-create-a-bootable-macos-sierra-installer-drive.html|How to create a bootable macOS Sierra installer drive]
'''OK let me clarify a few points:'''
You have the ability to install your OS directly from Apple via Internet Recovery which if the system was running El Capitan or newer will have the needed firmware update installed to allow this. All you need is directly access to the Internet. In addition your system should have a recovery partition setup during the OS install or update on your current drive. As long as the drive is functional you have that as well.
Here’s how! [https://support.apple.com/en-us/HT201255|Mac startup key combinations] & [https://support.apple.com/en-us/HT204904|How to reinstall macOS from macOS Recovery]
'''Now the fly in the ointment!'''
Apple allowed the newer macOS’s certificates expire prematurely! Here’s more on this [https://appleinsider.com/articles/19/10/24/if-youve-got-an-old-macos-install-image-it-will-probably-stop-working-today|If you've got an old macOS install image, it will probably stop working today]
I do recommend sticking with macOS Sierra as the best for your system. The issue is the limits of SATA drives. Apple upgraded the file system to APFS from the older HFS+ which runs better with SATA drives.
Here’s the updated installer [https://support.apple.com/en-us/HT208202|How to upgrade to macOS Sierra] jump down to Step 4 for the link.
To setup a bootable OS installer thumb drive follow this guide: [https://www.macworld.com/article/3092900/how-to-create-a-bootable-macos-sierra-installer-drive.html|How to create a bootable macOS Sierra installer drive]