I have a mobile hard disk in NTFS file format that can't be written on Mac

That's right, on Mac OS X, the NTFS file format partition (called the volume on the Mac) has no write access, only read access. It is very simple to write a partition in the NTFS file format, but it is dangerous. If it is not successful, you may lose access to the volume of the NTFS file format. The first method is to install MacFUSE. The method is extremely dangerous. As far as it is concerned, successful people seem to have not failed many people. Therefore, it is not recommended to install this software here. It is definitely not recommended for newcomers to try this software. The second method is relatively safe, but please also Prepare for failure! Install Paragon software. After installation, restart and read and write NTFS file format. However, if it is not successful, there will be a Paragon folder in the Application folder. There is Uninstaller program. Please run that program to uninstall Paragon instead of dragging the Paragon folder directly into the Trash or using the program uninstaller like AppZapper to try to uninstall. Also remind a very important thing: even Paragon, in order to ensure security, please be sure to repair the disk permissions in advance and restart before installing!