tom marchy Said:
"Installing Catalina on iMac currently on Sierra: Can I install Catalina on my iMac (27-inch, Late 2013) over my current MacOS Sierra, v10.12.6? Do I have to update the firmware first? Can I still boot and run off of an external drive containing my current MacOS Sierra, v10.12.6, after the Catalina upgrade on the internal drive?"
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Yes. Your Mac is compatible with Catalina.
So, install via the following:
- Backup you Mac.
- Upgrade to High Sierra with a USB Bootable Installer
- Manually Upgrade to Catalina.
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In Depth:
I. Back Up your Mac:
Create a Time Machine backup of your Mac, so that you can have something to restore your Mac from, should anything go wrong with the upgrade.
II. Upgrade to High Sierra with a USB Bootable Installer
Create a USB Bootable Installer:
- Go Here: How to create a bootable installer for macOS - Apple Support.
- Choose: the code for a High Sierra USB Bootable Installer
- Install: High Sierra from the USB Bootable Installer
Once Installed:
II. manually upgrade to Catalina.
Go Here: How to upgrade to macOS Catalina - Apple Support