Mount the hard drive first then navigate to the desired directories in it.

To find out the hard drive use this command in the terminal 

 fdisk -l

Then mount the hard drive partition using 

 mount /dev/partition_name /mnt

You can now list the directories for the mounted partition for checking by 

 ls /mnt


Then `cd` to the desired directory in the `/mnt` directory and delete the files using 

 rm -f file-name

I think this might solve the problem