MacOS – Looking for a free/cheap backup solution on OS X for NTFS external HDD

Install a copy of the OS X Server app on a spare Mac with a pile of storage connected to it. Create a share for each Mac that you need to back up, and connect each Mac to its own share. That way, each Mac only sees its own backups. You can also limit the size allowed for each share if you like.

Another advantage of having an OS X server on the network is that it can serve as a cache for downloaded updates from Apple, so each update will be downloaded only once, and stored on the cache.

There are other services that are available, and all you need is a spare Mac (I use a Mac Mini) to act as server, and a copy of the Server app from the App Store for $19.99 or so.

The community NTFS-3G driver in conjunction with Disk Utility doesn't allow you to repair the file system of an NTFS-formatted volume nor its permissions.

The file system repair feature is only available in the commercial Tuxera NTFS for Mac edition (based on the NTFS-3G driver) - either by using Disk Utility or Tuxera Disk Manager.

In the default configuration the files and folders of the mounted NTFS volume are owned by the effective user and group of the mounting process - the common settings are 777. This means all OS X users have full access to the volume or: Permissions are ignored!

At least in Linux using the community driver this can be overridden by modifying the mount options and user mappings (also: Advanced ownership and permissions). I don't know if this also works in OS X. The commercial Tuxera NTFS for Mac doesn't mention anything like this in its manual.