unable to paste to external hard drive mac

The problem is not with the Mac or the HDD.

Your HDD is of the Windows NTFS file system.
Open Disk Utility and format it and boot it with a system compatible with Mac. You will lose all your date but that is the safest option.

You can install Paragon for Mac OS and I believe that is a paid application but it overwrites the NTFS system and you will be able to write on the HDD.

Another option you have is to use a Sudo command in the terminal to manually overwrite it.

I opted with the Sudo command.
It worked well in the beginning but then after a while my HDD wasn't getting detected in the Finder window. But it wasn't a big issue I just had to go and undo the command. I would suggest you to keep all your things in one folder and use the Sudo command once a month. That's what I do.
Whenever my Macs storage is about to get half full, I empty everything into the HDD using the Sudo command in one go.

Googling for "using Sudo command to write on NTFS via Mac" will give you several links on how to do it.