I've had the same issue but I haven't upgraded to Big Sur yet - I'm on Catalina and 10.15.7 IOS with 2019 Macbook Pro that I bought new last December. I have a few external hard drives - 2 Seagates and a WD - the WD is partitioned for Time Machine and the other partition is for files I drag on it for easy access. One Seagate has movies and media on it while the other has mostly documents. I've done the disk utility thing - Was able to mount the WD but not the Time Machine side - which is frustrating - it's. 5TB drive! It can be read on my ****** PC and my old MacBook mid-2012 that is using High Sierra but not on my nearly brand MacBook pro. I feel like I need to go and buy another hard drive just to back up my newer Macbook - to be safe while the rest gets sorted out. I'm concerned about my computer backup on Time Machine - anyone else have this issue? I do not want to erase my other hard drives - The WD is pretty new too - only bought it a few months ago - I agree with everyone how frustrated I am with Apple - and though I'm pretty tech-savvy, I may just say the hell with it and go to the Genius bar to have them figure it out. I've wasted hours upon hours (days even) on this - time I don't have to spare. Funny how the drives can be seen on a ****** lenova pc but not on my expensive macbook pro. It seems crazy to have to create a new profile or erase perfectly good hard drives - this problem is with Apple, not the hard drives, or else I wouldn't be able to see them on my old Mac and my ****** PC.