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Catalina partition becomes unbootable and turns into "disk0s2"

I was dualbooting my MacBook with Catalina and Mavericks - install went perfectly fine, until I had to restart the computer, and discovered my Catalina partition is completely gone from the boot picker!

At first, I simply thought it was due to APFS not being recognized - however even with a Catalina install USB, it shows the Catalina partition as “disk0s2”, and I can’t write or install to it. Only the second Mavericks partition shows up. Using the diskutil list disk0 command in the terminal (lists partitions on disk0, the main drive in the computer) shows the same partition as FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF

It seems as if the data and the partition itself is still there, only it’s unable to be booted from. Now how do I boot from it? I apologize if the information given isn’t that much. Any help is really welcomed.

Edit - my Mavericks partition is still bootable. Only the Catalina partition - which doesn’t show up in the boot picker in the first place.

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What is the other OS in the other partition?

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@danj Mavericks. Sorry, just added it.

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Sounds like you had Catalina installed and then added Mavericks which messed up the boot partition. You’ll need to see if installing Catalina again fixes things.

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I can't get the Catalina installer to even install to "disk0s2"...

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@amatsuda223 - You'll need to setup an OS installer USB thumb drive after downloading the installer file from here How to get old versions of macOS and following this guide How to create a bootable macOS Catalina installer drive

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@danj I already have a Catalina install USB - my plan being to simply reinstall Catalina. It doesn't detect the partition.

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I've never tried installing an older OS-X onto a system with a newer macOS APFS release so I really don't know how to proceed without redoing the OS. I hope you have a backup ;-{

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@danj I managed to find this - https://apple.stackexchange.com/question... - seems it could help, unfortunately I got lost halfway through...

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amatsuda will be eternally grateful.
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