My HyperDrive 512 does not load the HD20 INIT at boot time. I guess that is why it cannot have HFS partitions. So without having checked the contents of my System Folder on that HyperDrive, I can only assume that the code required to make the HyperDrive boot is in the two ROM chips on the HyperDrive Controller Board.

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Then the HyperDrive works just like the MonsterMac except the MonsterMac patches SCSI code, whereas the HyperDrive is proprietary. It makes sense the HD20 doesn't load as the HyperDrive is was around long before the HD20 and is probably already patching into the same place. However didn't HyperDrive make versions for the Mac Plus? If so, I would imagine they might have created their own drive formatting utility and HD20-like INIT for patching HFS into their 512K retrofitted Macs.