I can not delete files by pressing 'Del' key on my keyboard. I don't know why that is. The files are mounted on an external drive that auto mounts at system startup. The drive can be mounted by any user, filesystem is NTFS.
I chose NTFS so I can access the files from a windows OS if needed.
all files have 777 permissions for root:root.
I can delete the file with right-click, delete. I can also delete the file using rm without sudo.
What I cannot do is select the file and press 'Del'.
the operating system is Ubuntu 16.04.
I do not have a professional answer but the only recommendations I could give you are the following: Disable Num Lock and try again, may be a problem in your keyboard. Also try Shift + Del keys and see what happens.
The second answer is more meaningful: you gave root permission in what OS? I you are running Linux and permissions were given by windows there will be a problem. I use a notebook with dual boot Win10 and Ubuntu but I could not delete a Windows file from Linux with the file manager and I could do this under the terminal.
At last, whether you gave root permissions under Linux you can execute this command. sudo chown -R $USER:$USER /media/your_external_hard_drive. $USER is your username under Linux.This command gave delete permissions from the file manager in your external hard drive.
This link will be useful whether you want to mount the external hard disk anytime. https://help.ubuntu.com/community/MountingWindowsPartitions
Regards,