Addendum...the main HD drive is also NOT mounted. I can't mount it, either. I know this is a simple fix but I don't know more...help!! Thanks...SSZ
PS It seems as if Big Sur was installed to the wrong (secondary) partition on the SSD so the secondary is now mounted and active. Another clue is that SuperDuper cannot find the previous primary partition. Any ideas on how to fix this, puh-leeeze!!?!