Tim's BigSur Mac is having issues reading an external hard drive. Leo says that since the drive is formatted to NTFS, Big Sur has trouble reading the drive. So it would be better if you reformatted the drive to APFS or ExFat or another Apple supportable file format. Also, try MacFuse or Paragon. That may help your Mac read the drive.
There is also a terminal command that can help. Read instructions here.
A discussion about it in Apple forums here.