Write locked HDD


I operate a WD My Passport Pro Wifi HDD (exFAT formatted) as a backup HDD on a windows 7 PC and an iPad (iOS 12.0). Access from Windows is via USB cable and Windows explorer. Access from iPad is via WD APP.
I am choosing the files manually that I want to backup and copy them to the external WD drive.
This worked well until today.
Now I do have read/write access from iPad but only read access from Windows.

Any idea what to do? Thanks!

Ok, I found it by myself:
obviously unmounting the HDD from Windows did fail and I unplugged the disk what caused an errorneous sector. Running chkdsk helped and I can access the drive again from Windows also with a write authorization.