If you let the beta upgrade your Fusion Drive to APFS, then you have to wipe and downgrade it back to HFS before you can upgrade to the GM - discu.eu

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If you let the beta upgrade your Fusion Drive to APFS, then you have to wipe and downgrade it back to HFS before you can upgrade to the GM https://beta.apple.com/sp/betaprogram/apfsfusion 18 comments 15/9/2017 apple