Solved ... for me at least ....Disk Utility -> First Aid I had a very similar issue with my brand new Macbook Air M1 with BigSur 11.4 I have two Seagate 8TB drives, both formatted as eXFAT, which worked perfectly on my 2015 Macbook Pro Retina and an aged Dell M6800 running Windows 10. No matter which drive, no matter how connected (Anker 6-in-1 PowerExpand usb-c hub. or another dongle usb-c to usb-3), I would ... a) not see the drive in Finder b) In Disk Utility the drive would be seen as not mounted, and would not mount manually c) powering up with the drive made no difference Following an Apple Tip in this thread, I then ran First Aid on the drive using Disk Utility, but... FIRST: attempt a Disk Utility -> Mount, wait a while and close the error message that says can't mount .... important step. Then Run First Aid ...... a) select the show details tick box so you can see what's going on then proceed b) Accept any "can't mount" or "unmount" messages if they appear c) after very few minutes the drive was "repaired", mounted, visible in Finder, and all files & folders seemed intact. d) continued to work when drive disconnected & reconnected Most importantly, after the "repair" the drive still worked perfectly on my old Macbook & PC I'll check out that the drive works with the other dongles and also do the same on my other 8TB drive. I've just done that with the same results.