If it is formatted as NTFS that could be the reason it is read only.
You have two alternatives either reformat to the file system native to macOS, HFS+, or buy third party software to allow the Mac to write to NTFS.
The reformat will require that any data you do not wish to lose will have to be copied somewhere else for the reformat.
Personal I recommend you reformat it.
Will doing this erase everything that I already have on the hard drive?
Yes. Reformatting a disk drive will remove all data from it.
Hence the reason I suggestion any data you do not wish to lose should be copied somewhere else.
External Hard drive is ready only?
