I'm also having this problem after kicking off an update to the latest Beta — the DTK chimes and reboots infinitely. I'm trying to follow the instructions at https://download.developer.apple.com/Documentation/Universal_App_Quick_Start_Program_Resources/Restoring_Developer_Transition_Kit.pdf but it's totally unclear where to obtain the IPSW for the latest Big Sur Beta, the instructions seem to simply assume that the file was previously downloaded or is available from https://developer.apple.com/download/, which is not the case AFAICT
jsilland
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