could not copy my files into the mounted external hard drive

Hi,
I am unable to copy my files to my external hard drive.

cp data.nc /run/media/wrf/My\ Passport/

cp: cannot create regular file i/run/media/wrf/My Passport/data.nci: Invalid argument

data.nc is about 8GB.

the following command gives,

lsblk -d -fs /dev/sdc1

NAME FSTYPE LABEL UUID
sdc1 ntfs My Passport 284EFC5C4EFC23EA

MOUNTPOINT
/run/media/wrf/My Passport/

I tried umount and mount the drive but I get following message

mount -t ntfs-3g /dev/sdc1 /run/media/wrf/My\ Passport/

Mount is denied because the NTFS volume is already exclusively opened.
The volume may be already mounted, or another software may use it which
could be identified for example by the help of the 'fuser' command

How can I mount the external hard drive to copy big files from server on it.

Thanking you in advance.

Sang