I'm having the exact same issue with a new 6Big. I can get the unit to work over USB 3.1, but not Thunderbolt 3. It appears to me that Big Sur is refusing to allow the driver to be installed, but in either case the 6Big DOES NOT WORK over Thunderbolt 3. The Mac Mini M1 doesn't even see it. Lacie's support is atrocious, I can't get ahold of anyone or get them to respond to emails. I think I'll return it.

LaCie support was zero help. There was effectively no trouble shooting whatsoever. They had me run a firmware update (it was already updated to the latest version), then check Privacy&Security to see if OS11 was preventing anything from running (when I first installed LaCie RAID Manager I got a warning saying OS11 blocked something from running, but there is now no mention of this in system preferences), then try a different Thunderbolt 3 cable. That was it. After that, LaCie said it should work, couldn't understand why it wasn't working, and that since I just purchased the unit I should return it. Somebody has to have figured this out by now, right? Please help!

I think I found what's going on. Under "Disabled Software", this is what appears. It looks like OS11 is preventing this from running. I've been on the phone with Apple support and they are blaming LaCie (which might be justified). Is there any way to allow this software to run, and see if it resolves the issue, or figure out why it was blocked? 7NPX47N25H - com.lacie.driver.mvumi: Version: 1.3.21 Reason: No User Consent Date Disabled: Unknown