Once started go to the menu to launch Disk Utility and then delete all of the partitions and containers. Once you've created the needed drive, connect it and then restart the system pressing the Option (⌥) key to get to the Startup Manager so you can select the thumb drive. Make sure to redo the format to GUID/HFS+ (Journaled) as the default is FAT32 on the thumb drives. You can use the built-in macOS Recovery to downgrade.
#Downgrade mac os without erasing how to
How to create a bootable macOS Catalina installer drive Assuming your Mac didnt come with the most recent version of macOS installed, downgrading is fairly easy.How to create a bootable macOS Mojave installer drive.How to create a bootable macOS High Sierra installer drive.How to create a bootable macOS Sierra installer drive.To repartition your hard drive, read this link. This option is available in Mac OS X 10.6 and higher as long as your machine supports the older operating system. In our case you really should stick with Sierra: You have one more option once backed up, before attempting a full downgrade.
Pick the OS you want to use from this list. As an example I have 6 setup now (of course using the correct OS for the given system). Problem being that I haven’t backed up, so would really like to do this without losing all of my files.
First, create an OS installer drive, You'll want to use a USB-A Thumb drive with a USB-A to USB-C dongle (that way you only need one set of thumb drives for the different OS's you want to use and can be used on the different systems. The guy in the Apple shop tells me that my machine is too old for the latest os and can’t handle it, so I need to downgrade to a previous os.