I'm in the same boat as as others since finally upgrading to Big Sur 11.3.1 on my MBP11,2 and can't get any of my external NTFS drives to be mounted on my MBP. IMHO... The worst thing to do is to format the drive "willynilly" without backing up all the contents. I'm a DJ and have over 2GB of songs that I am not just going to format and delete forever. The second worst thing to do is follow the above the above post as they must have a Hackintosh because their comments makes zero sense. I tried using my long time trusty app Mounty.app (v1.12) to mount my NTFS drives on Big Sur (worked amazingly on Catalina) but now I get an error "Volume "XXXX" not demountable. The volume is not re-mountable in read/write mode. Probably it was was not clean unmounted before." The best option I found is to back up the NTFS extrenal HD on a Windows OS 7-10 computer (if you can find one), then format the drive as exFAT on your Mac, then reinstall the files from the Windows computer back onto the external exFAT HD.