I have the same problem continuously ever since I upgraded to big Sur. I am currently on Version 11.4. I have tried everything including running First Aid on all my external drives (I have 3 one of them is an SSD), but nothing helps. It is so annoying with in the middle of my work, two drives drop off! These days it happens 3 or 4 times a day! SOme one suggested that I take Energy Saver off 'Default' for a little while it seemed to work now back to the same annoying Big Sur performance!

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.