USB issues with Big Sur

I am having the same problems. (just posted same question)
I do know for me my ext HD (Time Machine) did work. What happens with me is I connect the HD, systems crashes, then I don't do anything and let the computer reboot. For me the drive appears and I can let it resume its backup process.
I just downgraded back to 15.6 and wait to see if Apple corrects the problem.

I just updated my MacBook Pro to Big Sur and I'm having the same issue. My external SSD ejects not long after loading onto the desktop and clicking it. I'm getting a message that says, 'Eject "Extreme SSD" before disconnecting or turning it toff' and the SSD ejects. Can't do anything with it. I read that Catalina had the same bug which was fixed but wasn't fixed for Big Sur. Any help would be appreciated. I have all my client work on the ExSSD.

Thanks

I had no trouble with Catalina but I am having the same problem after I updated to Big Sur. Time Machine gives me a warning that I have lost connection with my external USB SSD HDD. When I turn the drive off and back on it is OK for a while. I have played with the Power setting for sleep, etc. and that makes no difference.

If you're using Western Digital or SanDisk drives, download and install the latest WD Discovery suite. Reboot (this is essential) and then connect each drive. Make sure UAS is OFF for each (gear icon next to the drive). Eject, and reconnect. Problem should be solved. It solved it for my WD SSD Passport external drives. Flying once again, no beach-balls, no crashing with scary data loss messages. You might need to reformat a Time Machine to get it to be stable in backing up (APFS format with GUID partitions, encrypted too if you want to keep it secure). Hope this helps!

If you're using Western Digital or SanDisk drives, download and install the latest WD Discovery suite. Reboot (this is essential) and then connect each drive. Make sure UAS is OFF for each (gear icon next to the drive). Eject, and reconnect. Problem should be solved. It solved it for my WD SSD Passport external drives. Flying once again, no beach-balls, no crashing with scary data loss messages. You might need to reformat a Time Machine to get it to be stable in backing up (APFS format with GUID partitions, encrypted too if you want to keep it secure). Hope this helps!

Same here

I have what appears to be the same problem with a Seagate Backup Plus 5TB:

• The disconnection occurs under heavy USB I/O (TimeMachine backup), and also simultaneously disconnects my WD Passport 2TB drive.

• Bluetooth devices (mouse and keyboard) are also disconnected, and the Bluetooth icon in the title bar has a line through it

• The desktop usually remains accessible via desktop sharing, when this happens, and the Bluetooth module seems to be switched off, and cannot be switched back on.

I can collect relevant logs, or other information, upon request.

I’m having the same issue with my two seagate 8tb drives in Raid 1 via CRU data port enclosure. I connect and everything is smooth for 2 minutes then Finder becomes unresponsive and I have to force restart. This wasn’t a problem in last OS. My Samsung T7 ssd is connecting and working fine though.

If you're using Western Digital or SanDisk drives, download and install the latest WD Discovery suite. Reboot (this is essential) and then connect each drive. Make sure UAS is OFF for each (gear icon next to the drive). Eject, and reconnect. Problem should be solved. It solved it for my WD SSD Passport external drives. Flying once again, no beach-balls, no crashing with scary data loss messages. You might need to reformat a Time Machine to get it to be stable in backing up (APFS format with GUID partitions, encrypted too if you want to keep it secure). Hope this helps!

If you're using Western Digital or SanDisk drives, download and install the latest WD Discovery suite. Reboot (this is essential) and then connect each drive. Make sure UAS is OFF for each (gear icon next to the drive). Eject, and reconnect. Problem should be solved. It solved it for my WD SSD Passport external drives. Flying once again, no beach-balls, no crashing with scary data loss messages. You might need to reformat a Time Machine to get it to be stable in backing up (APFS format with GUID partitions, encrypted too if you want to keep it secure). Hope this helps!

i also have exact same problem, i tried to mount the external hardesk in the utility but didn't work, and now i'm doing a first aid and its taking ages still didn't show any sign its going to work either!
and yes its only usb 3.0 devices that is not working anymore after the latest update BigSur, i tried everything i even formatted the laptop completely and stuck not able to have older version of the system so i had to reinstall bigsur! 
everything important is on that external hardisk all my work my art my health my everything its 1T its years and years .. i'm so sad for not being able to open it!
and every time i see a tutorial about downgrade it needs a usb! i couldn't find anything not required usb!
i won't give up i'll either figure a way to downgrade or fix the usb port by my self!
i'll be updating you!
but i'm very glad there is people like me in this situation because when i contacted apple support they told me this is their first time to hear about such a problem cause by BigSur!

now i know i'm not alone in this!

Have you try using Big Sur 11.4? And try using a USB-C to USB-A 3.0 converter. Or USB-C to USB-A cable? Does that work?

David Fraser's recommendation to switch off UAS on the Drive works. The problem is Big Sur. The Internet has thousands and thousands of people complaining about this issue. Just google to see how many people have the problem. Apple turns a blind eye to all the complaints, and claims it is not a Big Sur problem! Come on Apple, solve the problem.

I was having the same problem, USB Hard-Drive crashing all the time and having a severe performance issue.

I came across this post in another forum: https://apple.stackexchange.com/questions/176148/external-hard-drive-deep-freezing-macs

Go to settings - Spotlight - Privacy(right tab) and add the drive in question.

The OS stop crashing but I am still having some performance issues and the drive still ejects once in a while, but less frequently.