I have found that mounting hard drives is very slow on Big Sur. I have four external hard drives and it is about 2 minutes after login for these to appear on the desktop. I did some testing and a blank USB stick mounts instantly. An empty 2.5" USB spinning hard drive mounts instantly. I tried different formatting and encryption and it didn't make any real difference. It appears to be down to the contents of a drive. A blank drive is instant, but my time machine drive takes 30+ seconds. An external 2.5" drive with thousands of files can take a minute or more to appear. I presume macOS is running some sort of check on the contents? Is it possible to remove this check or shorten it?