I've got a WD My Passport 1TB SSD external drive. I also have 2 MacBook Airs, one mid 2013 and one less than a month old. I used the hard drive to transition from the old one to the new one. Both worked great.  I had the old one enrolled in the beta program so of course after moving to the new one it was too. They both upgraded to Big Sur one night. Long story short one of the programs I use regularly wasn't ready for Big Sur so I attempted to go back to Catalina. I plugged in the hard drive and it just sat there steadily flashing the light. No access in Finder.  I opened DiskUtility and there it was, greyed out with no access.. No mount, no first aid, nothing. So I opened Safari and did some looking. Meanwhile, I cleared out the old MacBook and loaded a new version of Catalina. Turns out, when I force quit fsckhfs, the drive is accessible and I can access it. While it passes all tests on WD's utility app, when I try to run first aid it fails after about 4 hours. Though I can still view and move files in finder, after the force quit of fsckhfs. If I try to renew from backup, I'm stuck because I can't force quit fsckhfs from that stage of boot yet I can't restore from backup after a full boot up. Should I attempt to erase the whole drive, reformat and start making new backups or is there a way to address the problem between the drive and fsckhfs? Interesting note, the drive worked perfectly with Catalina. After loading Big Sur it didn't work. After taking the older MacBook back from Big Sur to a fresh copy of Catalina, it didn't work anymore (unless I force quit fsck_hfs). FYI, the ext HD was connected to the old MacBook when it upgraded to Big Sur. The new one was never upgraded but the HD doesn't work when connected to the new one. Thanks to all for the assistance.