Ok - so here's what you need to do to avoid having to re-install clean!
1 - Boot into recovery console (cmd + r), open up a terminal
2 - spctl kext-consent add FC94733TZD
(FC94733TZD is the ATTO team ID)
3 - Reboot regularly, open a terminal
4 - sudo /sbin/kextload /Library/Extensions/ATTOiSCSI.kext
(At this point, you'll get the pop-up telling you to approve the system extension. Do this, and then it'll ask you for a reboot)
5 - Reboot - and live the Big Sur dream!