Areca wishes ArcMSR driver to remain in macOS to support external boot on Intel systems, and it will only be able to ship updates to this driver via future macOS updates.
ArcMSR has been moved to /System/Library/Extensions/ folder on the macOS Big Sur. When you upgraded your macOS to big sur beta 6, your TB Drive won't mount again.
Default driver ArcMSR V1.4.2 couldn't work. Notarized：No & Loaded：NO (Version 1.4.2 is Notarized and working on 10.15.x system). When you use Manual installation ArcMSR V1.4.2(same kext for Big Sur default) to the /Library/Extensions/ folder, the drive mounts again but after reboot the disk won't mount. Big Sur beta 6 displays two ArcMSR extensions on the System Information, both become Notarized：Yes, but still NO Loaded.
Areca has filed a bug to the http://bugreporter.apple.com/. The FB# is 8698185.
This issue has fixed on macOS Big Sur 11 Beta 10 (20A5395g).
Tags: ARC-8050T3, ArcMSR, driver, iMac