After buying a hard drive (Seagate Backup Hard Drive 5T) to backup my files, I was unable to copy them on it. Here a quick answer on why and how to fix it.
By default the hard drive use the Window file format called
Windows NT File System (NTFS)
It is then necessary to change the format to copy files on it with a mac.
On your mac go to:
Applications > Utilities > Disk Utility
the following window will then pop up
click on erase
select the format MAC OS Extended
and your are done you can now copy and paste files in the hard drive