Seeing the same issue on a Mac mini 2018, going from Catalina to Big Sur RC2.
Incredibly frustrating and a huge waste of everyone's time.
InkLondon
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Same issue here. Mac mini 2018 running Catalina 10.15.7.
Same issue at 12mins when trying to upgrade to Big Sur RC2.
This is going to be a big problem for Apple if they release this like this.