exFAT drive won't mount on Mac, fine on Windows. fsck_exfat gives "resource busy"

4TB external USB drive that reads / writes fine on Windows. It won't mount on Mac, and first aid via Disk Utility gives an error:

Filesystem check exit code is 1
Restoring the original state as found unmoutned
File system verify or repair failed (-69845)

Mounting via Disk Utility gives the error:

com.apple.DiskManagement error 0.

Booting into recovery mode, and running fsck_exfat -d diskxsx gives:

Can't open /dev/diskxsx: Resource Busy

Googling reveals similar problems but definitive answer that I can see? Any advice welcome.

Thanks!