OS X can't write in NTFS disks. If you want to write in NTFS drives, you have to use a third-party app like Paragon for NTFS. b
Another option is to format the external disk in exFAT on a PC, so you will be able to read and write in Macs and PCs without having to use third-party apps.
Or partition in two the external drive and use NTFS for the windows backups and Mac OS Extended file system for the Mac backups